Add Retro Shift (Auto Shift for Tap Hold via Retro Tapping) and Custom Auto Shifts (#11059)

* Add Retro Shift and Custom Auto Shifts

* Fix compilation errors with no RETRO_SHIFT value

6 files changed, 621 insertions, 125 deletions

diff --git a/docs/feature_auto_shift.md b/docs/feature_auto_shift.md
index a54cd79e4..99b0ca3c8 100644
--- a/docs/feature_auto_shift.md
+++ b/docs/feature_auto_shift.md
@@ -26,20 +26,26 @@ down will repeat the shifted key, though this can be disabled with
 once then immediately (within `TAPPING_TERM`) hold it down again (this works
 with the shifted value as well if auto-repeat is disabled).
 
+There are also the `get_auto_shift_repeat` and `get_auto_shift_no_auto_repeat`
+functions for more granular control. Neither will have an effect unless
+`AUTO_SHIFT_REPEAT_PER_KEY` or `AUTO_SHIFT_NO_AUTO_REPEAT_PER_KEY` respectively
+are defined.
+
 ## Are There Limitations to Auto Shift?
 
 Yes, unfortunately.
 
-You will have characters that are shifted when you did not intend on shifting, and
-other characters you wanted shifted, but were not. This simply comes down to
-practice. As we get in a hurry, we think we have hit the key long enough for a
-shifted version, but we did not. On the other hand, we may think we are tapping
-the keys, but really we have held it for a little longer than anticipated.
-
-Additionally, with keyrepeat the desired shift state can get mixed up. It will
-always 'belong' to the last key pressed. For example, keyrepeating a capital
-and then tapping something lowercase (whether or not it's an Auto Shift key)
-will result in the capital's *key* still being held, but shift not.
+1. You will have characters that are shifted when you did not intend on shifting, and
+   other characters you wanted shifted, but were not. This simply comes down to
+   practice. As we get in a hurry, we think we have hit the key long enough for a
+   shifted version, but we did not. On the other hand, we may think we are tapping
+   the keys, but really we have held it for a little longer than anticipated.
+2. Additionally, with keyrepeat the desired shift state can get mixed up. It will
+   always 'belong' to the last key pressed. For example, keyrepeating a capital
+   and then tapping something lowercase (whether or not it's an Auto Shift key)
+   will result in the capital's *key* still being held, but shift not.
+3. Auto Shift does not apply to Tap Hold keys. For automatic shifting of Tap Hold
+   keys see [Retro Shift](#retro-shift).
 
 ## How Do I Enable Auto Shift?
 
@@ -96,6 +102,34 @@ quicker than normal and you will be set.
 
 ?> Auto Shift has three special keys that can help you get this value right very quick. See "Auto Shift Setup" for more details!
 
+For more granular control of this feature, you can add the following to your `config.h`:
+
+```c
+#define AUTO_SHIFT_TIMEOUT_PER_KEY
+```
+
+You can then add the following function to your keymap:
+
+```c
+uint16_t get_autoshift_timeout(uint16_t keycode, keyrecord_t *record) {
+    switch(keycode) {
+        case AUTO_SHIFT_NUMERIC:
+            return 2 * get_generic_autoshift_timeout();
+        case AUTO_SHIFT_SPECIAL:
+            return get_generic_autoshift_timeout() + 50;
+        case AUTO_SHIFT_ALPHA:
+        default:
+            return get_generic_autoshift_timeout();
+    }
+}
+```
+
+Note that you cannot override individual keys that are in one of those groups
+if you are using them; trying to add a case for `KC_A` in the above example will
+not compile as `AUTO_SHIFT_ALPHA` is there. A possible solution is a second switch
+above to handle individual keys with no default case and only referencing the
+groups in the below fallback switch.
+
 ### NO_AUTO_SHIFT_SPECIAL (simple define)
 
 Do not Auto Shift special keys, which include -\_, =+, [{, ]}, ;:, '", ,<, .>,
@@ -109,11 +143,24 @@ Do not Auto Shift numeric keys, zero through nine.
 
 Do not Auto Shift alpha characters, which include A through Z.
 
-### Auto Shift Per Key 
+### Auto Shift Per Key
 
-This is a function that allows you to determine which keys shold be autoshifted, much like the tap-hold keys. 
+There are functions that allows you to determine which keys shold be autoshifted, much like the tap-hold keys.
 
-The default function looks like this: 
+The first of these, used to simply add a key to Auto Shift, is `get_custom_auto_shifted_key`:
+
+```c
+bool get_custom_auto_shifted_key(uint16_t keycode, keyrecord_t *record) {
+    switch(keycode) {
+        case KC_DOT:
+            return true;
+        default:
+            return false;
+    }
+}
+```
+
+For more granular control, there is `get_auto_shifted_key`. The default function looks like this:
 
 ```c
 bool get_auto_shifted_key(uint16_t keycode, keyrecord_t *record) {
@@ -131,9 +178,10 @@ bool get_auto_shifted_key(uint16_t keycode, keyrecord_t *record) {
 #    endif
             return true;
     }
-    return false;
+    return get_custom_auto_shifted_key(keycode, record);
 }
 ```
+
 This functionality is enabled by default, and does not need a define.
 
 ### AUTO_SHIFT_REPEAT (simple define)
@@ -144,6 +192,106 @@ Enables keyrepeat.
 
 Disables automatically keyrepeating when `AUTO_SHIFT_TIMEOUT` is exceeded.
 
+## Custom Shifted Values
+
+Especially on small keyboards, the default shifted value for many keys is not
+optimal. To provide more customizability, there are two user-definable
+functions, `autoshift_press/release_user`. These register or unregister the
+correct value for the passed key. Below is an example adding period to Auto
+Shift and making its shifted value exclamation point. Make sure to use weak
+mods - setting real would make any keys following it use their shifted values
+as if you were holding the key. Clearing of modifiers is handled by Auto Shift,
+and the OS-sent shift value if keyrepeating multiple keys is always that of
+the last key pressed (whether or not it's an Auto Shift key).
+
+You can also have non-shifted keys for the shifted values (or even no shifted
+value), just don't set a shift modifier!
+
+```c
+bool get_custom_auto_shifted_key(uint16_t keycode, keyrecord_t *record) {
+    switch(keycode) {
+        case KC_DOT:
+            return true;
+        default:
+            return false;
+    }
+}
+
+void autoshift_press_user(uint16_t keycode, bool shifted, keyrecord_t *record) {
+    switch(keycode) {
+        case KC_DOT:
+            register_code16((!shifted) ? KC_DOT : KC_EXLM);
+            break;
+        default:
+            if (shifted) {
+                add_weak_mods(MOD_BIT(KC_LSFT));
+            }
+            // & 0xFF gets the Tap key for Tap Holds, required when using Retro Shift
+            register_code16((IS_RETRO(keycode)) ? keycode & 0xFF : keycode);
+    }
+}
+
+void autoshift_release_user(uint16_t keycode, bool shifted, keyrecord_t *record) {
+    switch(keycode) {
+        case KC_DOT:
+            unregister_code16((!shifted) ? KC_DOT : KC_EXLM);
+            break;
+        default:
+            // & 0xFF gets the Tap key for Tap Holds, required when using Retro Shift
+            // The IS_RETRO check isn't really necessary here, always using
+            // keycode & 0xFF would be fine.
+            unregister_code16((IS_RETRO(keycode)) ? keycode & 0xFF : keycode);
+    }
+}
+```
+
+## Retro Shift
+
+Holding and releasing a Tap Hold key without pressing another key will ordinarily
+result in only the hold. With `retro shift` enabled this action will instead
+produce a shifted version of the tap keycode on release.
+
+It does not require [Retro Tapping](tap_hold.md#retro-tapping) to be enabled, and
+if both are enabled the state of `retro tapping` will only apply if the tap keycode
+is not matched by Auto Shift. `RETRO_TAPPING_PER_KEY` and its corresponding
+function, however, are checked before `retro shift` is applied.
+
+To enable `retro shift`, add the following to your `config.h`:
+
+```c
+#define RETRO_SHIFT
+```
+
+If `RETRO_SHIFT` is defined to a value, hold times greater than that value will
+not produce a tap on release for Mod Taps, and instead triggers the hold action.
+This enables modifiers to be held for combining with mouse clicks without
+generating taps on release. For example:
+
+```c
+#define RETRO_SHIFT 500
+```
+
+This value (if set) must be greater than one's `TAPPING_TERM`, as the key press
+must be designated as a 'hold' by `process_tapping` before we send the modifier.
+There is no such limitation in regards to `AUTO_SHIFT_TIMEOUT` for normal keys.
+
+### Retro Shift and Tap Hold Configurations
+
+Tap Hold Configurations work a little differently when using Retro Shift.
+Referencing `TAPPING_TERM` makes little sense, as holding longer would result in
+shifting one of the keys.
+
+`IGNORE_MOD_TAP_INTERRUPT` changes *only* rolling from a mod tap (releasing it
+first), sending both keys instead of the modifier on the second. Its effects on
+nested presses are ignored.
+
+As nested taps were changed to act as though `PERMISSIVE_HOLD` is set unless only
+`IGNORE_MOD_TAP_INTERRUPT` is (outside of Retro Shift), and Retro Shift ignores
+`IGNORE_MOD_TAP_INTERRUPT`, `PERMISSIVE_HOLD` has no effect on Mod Taps.
+
+Nested taps will *always* act as though the `TAPPING_TERM` was exceeded for both
+Mod and Layer Tap keys.
+
 ## Using Auto Shift Setup
 
 This will enable you to define three keys temporarily to increase, decrease and report your `AUTO_SHIFT_TIMEOUT`.
diff --git a/docs/tap_hold.md b/docs/tap_hold.md
index dbad48fd9..a343d0bc3 100644
--- a/docs/tap_hold.md
+++ b/docs/tap_hold.md
@@ -268,6 +268,10 @@ bool get_retro_tapping(uint16_t keycode, keyrecord_t *record) {
 }
 ```
 
+### Retro Shift
+
+[Auto Shift,](feature_auto_shift.md) has its own version of `retro tapping` called `retro shift`. It is extremely similar to `retro tapping`, but holding the key past `AUTO_SHIFT_TIMEOUT` results in the value it sends being shifted. Other configurations also affect it differently; see [here](feature_auto_shift.md#retro-shift) for more information.
+
 ## Why do we include the key record for the per key functions?
 
 One thing that you may notice is that we include the key record for all of the "per key" functions, and may be wondering why we do that.
diff --git a/quantum/action.c b/quantum/action.c
index ceaaa551f..5e81efb67 100644
--- a/quantum/action.c
+++ b/quantum/action.c
@@ -45,10 +45,14 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 int tp_buttons;
 
-#if defined(RETRO_TAPPING) || defined(RETRO_TAPPING_PER_KEY)
+#if defined(RETRO_TAPPING) || defined(RETRO_TAPPING_PER_KEY) || (defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT))
 int retro_tapping_counter = 0;
 #endif
 
+#if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT) && !defined(NO_ACTION_TAPPING)
+#    include "process_auto_shift.h"
+#endif
+
 #ifdef IGNORE_MOD_TAP_INTERRUPT_PER_KEY
 __attribute__((weak)) bool get_ignore_mod_tap_interrupt(uint16_t keycode, keyrecord_t *record) { return false; }
 #endif
@@ -69,7 +73,7 @@ void action_exec(keyevent_t event) {
         dprint("EVENT: ");
         debug_event(event);
         dprintln();
-#if defined(RETRO_TAPPING) || defined(RETRO_TAPPING_PER_KEY)
+#if defined(RETRO_TAPPING) || defined(RETRO_TAPPING_PER_KEY) || (defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT))
         retro_tapping_counter++;
 #endif
     }
@@ -106,6 +110,11 @@ void action_exec(keyevent_t event) {
 #endif
 
 #ifndef NO_ACTION_TAPPING
+#    if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)
+    if (event.pressed) {
+        retroshift_poll_time(&event);
+    }
+#    endif
     if (IS_NOEVENT(record.event) || pre_process_record_quantum(&record)) {
         action_tapping_process(record);
     }
@@ -730,7 +739,7 @@ void process_action(keyrecord_t *record, action_t action) {
 #endif
 
 #ifndef NO_ACTION_TAPPING
-#    if defined(RETRO_TAPPING) || defined(RETRO_TAPPING_PER_KEY)
+#    if defined(RETRO_TAPPING) || defined(RETRO_TAPPING_PER_KEY) || (defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT))
     if (!is_tap_action(action)) {
         retro_tapping_counter = 0;
     } else {
@@ -747,7 +756,11 @@ void process_action(keyrecord_t *record, action_t action) {
                     get_retro_tapping(get_event_keycode(record->event, false), record) &&
 #        endif
                     retro_tapping_counter == 2) {
+#        if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)
+                    process_auto_shift(action.layer_tap.code, record);
+#        else
                     tap_code(action.layer_tap.code);
+#        endif
                 }
                 retro_tapping_counter = 0;
             }
diff --git a/quantum/action_tapping.c b/quantum/action_tapping.c
index 60e56fb81..0586fad42 100644
--- a/quantum/action_tapping.c
+++ b/quantum/action_tapping.c
@@ -44,6 +44,10 @@ __attribute__((weak)) bool get_permissive_hold(uint16_t keycode, keyrecord_t *re
 __attribute__((weak)) bool get_hold_on_other_key_press(uint16_t keycode, keyrecord_t *record) { return false; }
 #    endif
 
+#    if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)
+#        include "process_auto_shift.h"
+#    endif
+
 static keyrecord_t tapping_key                         = {};
 static keyrecord_t waiting_buffer[WAITING_BUFFER_SIZE] = {};
 static uint8_t     waiting_buffer_head                 = 0;
@@ -107,12 +111,29 @@ void action_tapping_process(keyrecord_t record) {
 /* return true when key event is processed or consumed. */
 bool process_tapping(keyrecord_t *keyp) {
     keyevent_t event = keyp->event;
+#    if (defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)) || defined(TAPPING_TERM_PER_KEY) || defined(PERMISSIVE_HOLD_PER_KEY) || defined(TAPPING_FORCE_HOLD_PER_KEY) || defined(HOLD_ON_OTHER_KEYPRESS_PER_KEY)
+    uint16_t tapping_keycode = get_record_keycode(&tapping_key, false);
+#    endif
 
     // if tapping
     if (IS_TAPPING_PRESSED()) {
-        if (WITHIN_TAPPING_TERM(event)) {
+        // clang-format off
+        if (WITHIN_TAPPING_TERM(event)
+#    if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)
+            || (
+#        ifdef RETRO_TAPPING_PER_KEY
+                get_retro_tapping(tapping_keycode, keyp) &&
+#        endif
+                (RETRO_SHIFT + 0) != 0 && TIMER_DIFF_16(event.time, tapping_key.event.time) < (RETRO_SHIFT + 0)
+            )
+#    endif
+        ) {
+            // clang-format on
             if (tapping_key.tap.count == 0) {
                 if (IS_TAPPING_RECORD(keyp) && !event.pressed) {
+#    if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)
+                    retroshift_swap_times();
+#    endif
                     // first tap!
                     debug("Tapping: First tap(0->1).\n");
                     tapping_key.tap.count = 1;
@@ -128,22 +149,70 @@ bool process_tapping(keyrecord_t *keyp) {
                  * This can register the key before settlement of tapping,
                  * useful for long TAPPING_TERM but may prevent fast typing.
                  */
-#    if defined(TAPPING_TERM_PER_KEY) || (TAPPING_TERM >= 500) || defined(PERMISSIVE_HOLD) || defined(PERMISSIVE_HOLD_PER_KEY)
-                else if (((
+                // clang-format off
+#    if defined(TAPPING_TERM_PER_KEY) || (TAPPING_TERM >= 500) || defined(PERMISSIVE_HOLD) || defined(PERMISSIVE_HOLD_PER_KEY) || (defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT))
+                else if (
+                    (
+                        (
+                            (
 #        ifdef TAPPING_TERM_PER_KEY
-                              get_tapping_term(get_record_keycode(&tapping_key, false), keyp)
+                                get_tapping_term(tapping_keycode, keyp)
 #        else
-                              TAPPING_TERM
+                                TAPPING_TERM
 #        endif
-                              >= 500)
+                                >= 500
+                            )
 
 #        ifdef PERMISSIVE_HOLD_PER_KEY
-                          || get_permissive_hold(get_record_keycode(&tapping_key, false), keyp)
+                            || get_permissive_hold(tapping_keycode, keyp)
 #        elif defined(PERMISSIVE_HOLD)
-                          || true
+                            || true
+#        endif
+                        ) && IS_RELEASED(event) && waiting_buffer_typed(event)
+                    )
+                    // Causes nested taps to not wait past TAPPING_TERM/RETRO_SHIFT
+                    // unnecessarily and fixes them for Layer Taps.
+#        if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)
+                    || (
+#            ifdef RETRO_TAPPING_PER_KEY
+                        get_retro_tapping(tapping_keycode, keyp) &&
+#            endif
+                        (
+                            // Rolled over the two keys.
+                            (
+                                (
+                                    false
+#            if defined(HOLD_ON_OTHER_KEYPRESS) || defined(HOLD_ON_OTHER_KEYPRESS_PER_KEY)
+                                    || (
+                                        IS_LT(tapping_keycode)
+#                ifdef HOLD_ON_OTHER_KEYPRESS_PER_KEY
+                                        && get_hold_on_other_keypress(tapping_keycode, keyp)
+#                endif
+                                    )
+#            endif
+#            if !defined(IGNORE_MOD_TAP_INTERRUPT) || defined(IGNORE_MOD_TAP_INTERRUPT_PER_KEY)
+                                    || (
+                                        IS_MT(tapping_keycode)
+#                ifdef IGNORE_MOD_TAP_INTERRUPT_PER_KEY
+                                        && !get_ignore_mod_tap_interrupt(tapping_keycode, keyp)
+#                endif
+                                    )
+#            endif
+                                ) && tapping_key.tap.interrupted == true
+                            )
+                            // Makes Retro Shift ignore [IGNORE_MOD_TAP_INTERRUPT's
+                            // effects on nested taps for MTs and the default
+                            // behavior of LTs] below TAPPING_TERM or RETRO_SHIFT.
+                            || (
+                                IS_RETRO(tapping_keycode)
+                                && (event.key.col != tapping_key.event.key.col || event.key.row != tapping_key.event.key.row)
+                                && IS_RELEASED(event) && waiting_buffer_typed(event)
+                            )
+                        )
+                    )
 #        endif
-                              ) &&
-                         IS_RELEASED(event) && waiting_buffer_typed(event)) {
+                ) {
+                    // clang-format on
                     debug("Tapping: End. No tap. Interfered by typing key\n");
                     process_record(&tapping_key);
                     tapping_key = (keyrecord_t){};
@@ -181,7 +250,7 @@ bool process_tapping(keyrecord_t *keyp) {
                         tapping_key.tap.interrupted = true;
 #    if defined(HOLD_ON_OTHER_KEY_PRESS) || defined(HOLD_ON_OTHER_KEY_PRESS_PER_KEY)
 #        if defined(HOLD_ON_OTHER_KEY_PRESS_PER_KEY)
-                        if (get_hold_on_other_key_press(get_record_keycode(&tapping_key, false), keyp))
+                        if (get_hold_on_other_key_press(tapping_keycode, keyp))
 #        endif
                         {
                             debug("Tapping: End. No tap. Interfered by pressed key\n");
@@ -284,14 +353,25 @@ bool process_tapping(keyrecord_t *keyp) {
             }
         }
     } else if (IS_TAPPING_RELEASED()) {
-        if (WITHIN_TAPPING_TERM(event)) {
+        // clang-format off
+        if (WITHIN_TAPPING_TERM(event)
+#    if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)
+            || (
+#        ifdef RETRO_TAPPING_PER_KEY
+                get_retro_tapping(tapping_keycode, keyp) &&
+#        endif
+                (RETRO_SHIFT + 0) != 0 && TIMER_DIFF_16(event.time, tapping_key.event.time) < (RETRO_SHIFT + 0)
+            )
+#    endif
+        ) {
+            // clang-format on
             if (event.pressed) {
                 if (IS_TAPPING_RECORD(keyp)) {
 //#    ifndef TAPPING_FORCE_HOLD
 #    if !defined(TAPPING_FORCE_HOLD) || defined(TAPPING_FORCE_HOLD_PER_KEY)
                     if (
 #        ifdef TAPPING_FORCE_HOLD_PER_KEY
-                        !get_tapping_force_hold(get_record_keycode(&tapping_key, false), keyp) &&
+                        !get_tapping_force_hold(tapping_keycode, keyp) &&
 #        endif
                         !tapping_key.tap.interrupted && tapping_key.tap.count > 0) {
                         // sequential tap.
diff --git a/quantum/process_keycode/process_auto_shift.c b/quantum/process_keycode/process_auto_shift.c
index 4d928edb5..bbc6367ff 100644
--- a/quantum/process_keycode/process_auto_shift.c
+++ b/quantum/process_keycode/process_auto_shift.c
@@ -18,7 +18,6 @@
 
 #    include <stdbool.h>
 #    include <stdio.h>
-
 #    include "process_auto_shift.h"
 
 #    ifndef AUTO_SHIFT_DISABLED_AT_STARTUP
@@ -27,11 +26,25 @@
 #        define AUTO_SHIFT_STARTUP_STATE false /* disabled */
 #    endif
 
-static uint16_t autoshift_time    = 0;
-static uint16_t autoshift_timeout = AUTO_SHIFT_TIMEOUT;
-static uint16_t autoshift_lastkey = KC_NO;
+// Stores the last Auto Shift key's up or down time, for evaluation or keyrepeat.
+static uint16_t autoshift_time = 0;
+#    if defined(RETRO_SHIFT) && !defined(NO_ACTION_TAPPING)
+// Stores the last key's up or down time, to replace autoshift_time so that Tap Hold times are accurate.
+static uint16_t retroshift_time = 0;
+// Stores a possibly Retro Shift key's up or down time, as retroshift_time needs
+// to be set before the Retro Shift key is evaluated if it is interrupted by an
+// Auto Shifted key.
+static uint16_t last_retroshift_time;
+#    endif
+static uint16_t    autoshift_timeout = AUTO_SHIFT_TIMEOUT;
+static uint16_t    autoshift_lastkey = KC_NO;
+static keyrecord_t autoshift_lastrecord;
+// Keys take 8 bits if modifiers are excluded. This records the shift state
+// when pressed for each key, so that can be passed to the release function
+// and it knows which key needs to be released (if shifted is different base).
+static uint16_t autoshift_shift_states[((1 << 8) + 15) / 16];
 static struct {
-    // Whether autoshift is enabled.
+    // Whether Auto Shift is enabled.
     bool enabled : 1;
     // Whether the last auto-shifted key was released after the timeout.  This
     // is used to replicate the last key for a tap-then-hold.
@@ -40,43 +53,157 @@ static struct {
     bool in_progress : 1;
     // Whether the auto-shifted keypress has been registered.
     bool holding_shift : 1;
-} autoshift_flags = {AUTO_SHIFT_STARTUP_STATE, false, false, false};
+    // Whether the user is holding a shift and we removed it.
+    bool cancelling_lshift : 1;
+    bool cancelling_rshift : 1;
+    // clang-format wants to remove the true for some reason.
+    // clang-format off
+} autoshift_flags = {AUTO_SHIFT_STARTUP_STATE, false, false, false, false, false};
+// clang-format on
+
+/** \brief Called on physical press, returns whether key should be added to Auto Shift */
+__attribute__((weak)) bool get_custom_auto_shifted_key(uint16_t keycode, keyrecord_t *record) { return false; }
+
+/** \brief Called on physical press, returns whether is Auto Shift key */
+__attribute__((weak)) bool get_auto_shifted_key(uint16_t keycode, keyrecord_t *record) {
+    switch (keycode) {
+#    ifndef NO_AUTO_SHIFT_ALPHA
+        case AUTO_SHIFT_ALPHA:
+#    endif
+#    ifndef NO_AUTO_SHIFT_NUMERIC
+        case AUTO_SHIFT_NUMERIC:
+#    endif
+#    ifndef NO_AUTO_SHIFT_SPECIAL
+        case AUTO_SHIFT_SPECIAL:
+#    endif
+            return true;
+    }
+    return get_custom_auto_shifted_key(keycode, record);
+}
+
+/** \brief Called to check whether defines should apply if PER_KEY is set for it */
+__attribute__((weak)) bool get_auto_shift_repeat(uint16_t keycode, keyrecord_t *record) { return true; }
+__attribute__((weak)) bool get_auto_shift_no_auto_repeat(uint16_t keycode, keyrecord_t *record) { return true; }
+
+/** \brief Called when an Auto Shift key needs to be pressed */
+__attribute__((weak)) void autoshift_press_user(uint16_t keycode, bool shifted, keyrecord_t *record) {
+    if (shifted) {
+        add_weak_mods(MOD_BIT(KC_LSFT));
+    }
+    register_code16((IS_RETRO(keycode)) ? keycode & 0xFF : keycode);
+}
+
+/** \brief Called when an Auto Shift key needs to be released */
+__attribute__((weak)) void autoshift_release_user(uint16_t keycode, bool shifted, keyrecord_t *record) { unregister_code16((IS_RETRO(keycode)) ? keycode & 0xFF : keycode); }
+
+/** \brief Sets the shift state to use when keyrepeating, required by custom shifts */
+void set_autoshift_shift_state(uint16_t keycode, bool shifted) {
+    keycode = keycode & 0xFF;
+    if (shifted) {
+        autoshift_shift_states[keycode / 16] |= (uint16_t)1 << keycode % 16;
+    } else {
+        autoshift_shift_states[keycode / 16] &= ~((uint16_t)1 << keycode % 16);
+    }
+}
+
+/** \brief Gets the shift state to use when keyrepeating, required by custom shifts */
+bool get_autoshift_shift_state(uint16_t keycode) {
+    keycode = keycode & 0xFF;
+    return (autoshift_shift_states[keycode / 16] & (uint16_t)1 << keycode % 16) != (uint16_t)0;
+}
+
+/** \brief Restores the shift key if it was cancelled by Auto Shift */
+static void autoshift_flush_shift(void) {
+    autoshift_flags.holding_shift = false;
+    del_weak_mods(MOD_BIT(KC_LSFT));
+    if (autoshift_flags.cancelling_lshift) {
+        autoshift_flags.cancelling_lshift = false;
+        add_mods(MOD_BIT(KC_LSFT));
+    }
+    if (autoshift_flags.cancelling_rshift) {
+        autoshift_flags.cancelling_rshift = false;
+        add_mods(MOD_BIT(KC_RSFT));
+    }
+    send_keyboard_report();
+}
 
 /** \brief Record the press of an autoshiftable key
  *
  *  \return Whether the record should be further processed.
  */
 static bool autoshift_press(uint16_t keycode, uint16_t now, keyrecord_t *record) {
-    if (!autoshift_flags.enabled) {
-        return true;
-    }
-
+    // clang-format off
+    if ((get_mods()
+#    if !defined(NO_ACTION_ONESHOT) && !defined(NO_ACTION_TAPPING)
+            | get_oneshot_mods()
+#    endif
+        ) & (~MOD_BIT(KC_LSFT))
+    ) {
+        // clang-format on
+        // Prevents keyrepeating unshifted value of key after using it in a key combo.
+        autoshift_lastkey = KC_NO;
 #    ifndef AUTO_SHIFT_MODIFIERS
-    if (get_mods()) {
-        return true;
-    }
+        // We can't return true here anymore because custom unshifted values are
+        // possible and there's no good way to tell whether the press returned
+        // true upon release.
+        set_autoshift_shift_state(keycode, false);
+        autoshift_press_user(keycode, false, record);
+#        if !defined(NO_ACTION_ONESHOT) && !defined(NO_ACTION_TAPPING)
+        set_oneshot_mods(get_oneshot_mods() & (~MOD_BIT(KC_LSFT)));
+        clear_oneshot_layer_state(ONESHOT_OTHER_KEY_PRESSED);
+#        endif
+        return false;
 #    endif
-#    ifdef AUTO_SHIFT_REPEAT
-    const uint16_t elapsed = TIMER_DIFF_16(now, autoshift_time);
-#        ifndef AUTO_SHIFT_NO_AUTO_REPEAT
-    if (!autoshift_flags.lastshifted) {
+    }
+
+    // Store record to be sent to user functions if there's no release record then.
+    autoshift_lastrecord               = *record;
+    autoshift_lastrecord.event.pressed = false;
+    autoshift_lastrecord.event.time    = 0;
+    // clang-format off
+#    if defined(AUTO_SHIFT_REPEAT) || defined(AUTO_SHIFT_REPEAT_PER_KEY)
+    if (keycode == autoshift_lastkey &&
+#        ifdef AUTO_SHIFT_REPEAT_PER_KEY
+        get_auto_shift_repeat(autoshift_lastkey, record) &&
 #        endif
-        if (elapsed < TAPPING_TERM && keycode == autoshift_lastkey) {
-            // Allow a tap-then-hold for keyrepeat.
-            if (!autoshift_flags.lastshifted) {
-                register_code(autoshift_lastkey);
-            } else {
-                // Simulate pressing the shift key.
-                add_weak_mods(MOD_BIT(KC_LEFT_SHIFT));
-                register_code(autoshift_lastkey);
-            }
-            return false;
+#        if !defined(AUTO_SHIFT_NO_AUTO_REPEAT) || defined(AUTO_SHIFT_NO_AUTO_REPEAT_PER_KEY)
+        (
+            !autoshift_flags.lastshifted
+#            ifdef AUTO_SHIFT_NO_AUTO_REPEAT_PER_KEY
+            || get_auto_shift_no_auto_repeat(autoshift_lastkey, record)
+#            endif
+        ) &&
+#        endif
+        TIMER_DIFF_16(now, autoshift_time) <
+#        ifdef TAPPING_TERM_PER_KEY
+        get_tapping_term(autoshift_lastkey, record)
+#        else
+        TAPPING_TERM
+#        endif
+    ) {
+        // clang-format on
+        // Allow a tap-then-hold for keyrepeat.
+        if (get_mods() & MOD_BIT(KC_LSFT)) {
+            autoshift_flags.cancelling_lshift = true;
+            del_mods(MOD_BIT(KC_LSFT));
         }
-#        ifndef AUTO_SHIFT_NO_AUTO_REPEAT
+        if (get_mods() & MOD_BIT(KC_RSFT)) {
+            autoshift_flags.cancelling_rshift = true;
+            del_mods(MOD_BIT(KC_RSFT));
+        }
+        // autoshift_shift_state doesn't need to be changed.
+        autoshift_press_user(autoshift_lastkey, autoshift_flags.lastshifted, record);
+        return false;
     }
-#        endif
 #    endif
 
+    // Use physical shift state of press event to be more like normal typing.
+#    if !defined(NO_ACTION_ONESHOT) && !defined(NO_ACTION_TAPPING)
+    autoshift_flags.lastshifted = (get_mods() | get_oneshot_mods()) & MOD_BIT(KC_LSFT);
+    set_oneshot_mods(get_oneshot_mods() & (~MOD_BIT(KC_LSFT)));
+#    else
+    autoshift_flags.lastshifted = get_mods() & MOD_BIT(KC_LSFT);
+#    endif
     // Record the keycode so we can simulate it later.
     autoshift_lastkey           = keycode;
     autoshift_time              = now;
@@ -90,51 +217,70 @@ static bool autoshift_press(uint16_t keycode, uint16_t now, keyrecord_t *record)
 
 /** \brief Registers an autoshiftable key under the right conditions
  *
- * If the autoshift delay has elapsed, register a shift and the key.
+ * If autoshift_timeout has elapsed, register a shift and the key.
  *
- * If the autoshift key is released before the delay has elapsed, register the
+ * If the Auto Shift key is released before the delay has elapsed, register the
  * key without a shift.
+ *
+ * Called on key down with keycode=KC_NO, auto-shifted key up, and timeout.
  */
-static void autoshift_end(uint16_t keycode, uint16_t now, bool matrix_trigger) {
-    // Called on key down with KC_NO, auto-shifted key up, and timeout.
-    if (autoshift_flags.in_progress) {
+static void autoshift_end(uint16_t keycode, uint16_t now, bool matrix_trigger, keyrecord_t *record) {
+    if (autoshift_flags.in_progress && (keycode == autoshift_lastkey || keycode == KC_NO)) {
         // Process the auto-shiftable key.
         autoshift_flags.in_progress = false;
-
-        // Time since the initial press was recorded.
-        const uint16_t elapsed = TIMER_DIFF_16(now, autoshift_time);
-        if (elapsed < autoshift_timeout) {
-            register_code(autoshift_lastkey);
-            autoshift_flags.lastshifted = false;
-        } else {
-            // Simulate pressing the shift key.
-            add_weak_mods(MOD_BIT(KC_LEFT_SHIFT));
-            register_code(autoshift_lastkey);
-            autoshift_flags.lastshifted = true;
-#    if defined(AUTO_SHIFT_REPEAT) && !defined(AUTO_SHIFT_NO_AUTO_REPEAT)
-            if (matrix_trigger) {
-                // Prevents release.
-                return;
-            }
+        // clang-format off
+        autoshift_flags.lastshifted =
+            autoshift_flags.lastshifted
+            || TIMER_DIFF_16(now, autoshift_time) >=
+#    ifdef AUTO_SHIFT_TIMEOUT_PER_KEY
+                get_autoshift_timeout(autoshift_lastkey, record)
+#    else
+                autoshift_timeout
 #    endif
+        ;
+        // clang-format on
+        set_autoshift_shift_state(autoshift_lastkey, autoshift_flags.lastshifted);
+        if (get_mods() & MOD_BIT(KC_LSFT)) {
+            autoshift_flags.cancelling_lshift = true;
+            del_mods(MOD_BIT(KC_LSFT));
+        }
+        if (get_mods() & MOD_BIT(KC_RSFT)) {
+            autoshift_flags.cancelling_rshift = true;
+            del_mods(MOD_BIT(KC_RSFT));
         }
+        autoshift_press_user(autoshift_lastkey, autoshift_flags.lastshifted, record);
 
+        // clang-format off
+#    if (defined(AUTO_SHIFT_REPEAT) || defined(AUTO_SHIFT_REPEAT_PER_KEY)) && (!defined(AUTO_SHIFT_NO_AUTO_REPEAT) || defined(AUTO_SHIFT_NO_AUTO_REPEAT_PER_KEY))
+        if (matrix_trigger
+#        ifdef AUTO_SHIFT_REPEAT_PER_KEY
+            && get_auto_shift_repeat(autoshift_lastkey, record)
+#        endif
+#        ifdef AUTO_SHIFT_NO_AUTO_REPEAT_PER_KEY
+            && !get_auto_shift_no_auto_repeat(autoshift_lastkey, record)
+#        endif
+        ) {
+            // Prevents release.
+            return;
+        }
+#    endif
+        // clang-format on
 #    if TAP_CODE_DELAY > 0
         wait_ms(TAP_CODE_DELAY);
 #    endif
-        unregister_code(autoshift_lastkey);
-        del_weak_mods(MOD_BIT(KC_LEFT_SHIFT));
+
+        autoshift_release_user(autoshift_lastkey, autoshift_flags.lastshifted, record);
+        autoshift_flush_shift();
     } else {
         // Release after keyrepeat.
-        unregister_code(keycode);
+        autoshift_release_user(keycode, get_autoshift_shift_state(keycode), record);
         if (keycode == autoshift_lastkey) {
             // This will only fire when the key was the last auto-shiftable
-            // pressed. That prevents aaaaBBBB then releasing a from unshifting
-            // later Bs (if B wasn't auto-shiftable).
-            del_weak_mods(MOD_BIT(KC_LEFT_SHIFT));
+            // pressed. That prevents 'aaaaBBBB' then releasing a from unshifting
+            // later 'B's (if 'B' wasn't auto-shiftable).
+            autoshift_flush_shift();
         }
     }
-    send_keyboard_report();  // del_weak_mods doesn't send one.
     // Roll the autoshift_time forward for detecting tap-and-hold.
     autoshift_time = now;
 }
@@ -147,24 +293,29 @@ static void autoshift_end(uint16_t keycode, uint16_t now, bool matrix_trigger) {
  */
 void autoshift_matrix_scan(void) {
     if (autoshift_flags.in_progress) {
-        const uint16_t now     = timer_read();
-        const uint16_t elapsed = TIMER_DIFF_16(now, autoshift_time);
-        if (elapsed >= autoshift_timeout) {
-            autoshift_end(autoshift_lastkey, now, true);
+        const uint16_t now = timer_read();
+        if (TIMER_DIFF_16(now, autoshift_time) >=
+#    ifdef AUTO_SHIFT_TIMEOUT_PER_KEY
+            get_autoshift_timeout(autoshift_lastkey, &autoshift_lastrecord)
+#    else
+            autoshift_timeout
+#    endif
+        ) {
+            autoshift_end(autoshift_lastkey, now, true, &autoshift_lastrecord);
         }
     }
 }
 
 void autoshift_toggle(void) {
     autoshift_flags.enabled = !autoshift_flags.enabled;
-    del_weak_mods(MOD_BIT(KC_LEFT_SHIFT));
+    autoshift_flush_shift();
 }
 
 void autoshift_enable(void) { autoshift_flags.enabled = true; }
 
 void autoshift_disable(void) {
     autoshift_flags.enabled = false;
-    del_weak_mods(MOD_BIT(KC_LEFT_SHIFT));
+    autoshift_flush_shift();
 }
 
 #    ifndef AUTO_SHIFT_NO_SETUP
@@ -179,76 +330,158 @@ void autoshift_timer_report(void) {
 
 bool get_autoshift_state(void) { return autoshift_flags.enabled; }
 
-uint16_t get_autoshift_timeout(void) { return autoshift_timeout; }
+uint16_t                       get_generic_autoshift_timeout() { return autoshift_timeout; }
+__attribute__((weak)) uint16_t get_autoshift_timeout(uint16_t keycode, keyrecord_t *record) { return autoshift_timeout; }
 
 void set_autoshift_timeout(uint16_t timeout) { autoshift_timeout = timeout; }
 
 bool process_auto_shift(uint16_t keycode, keyrecord_t *record) {
     // Note that record->event.time isn't reliable, see:
     // https://github.com/qmk/qmk_firmware/pull/9826#issuecomment-733559550
-    const uint16_t now = timer_read();
+    // clang-format off
+    const uint16_t now =
+#    if !defined(RETRO_SHIFT) || defined(NO_ACTION_TAPPING)
+        timer_read()
+#    else
+        (record->event.pressed) ? retroshift_time : timer_read()
+#    endif
+    ;
+    // clang-format on
 
     if (record->event.pressed) {
         if (autoshift_flags.in_progress) {
-            // Evaluate previous key if there is one. Doing this elsewhere is
-            // more complicated and easier to break.
-            autoshift_end(KC_NO, now, false);
+            // Evaluate previous key if there is one.
+            autoshift_end(KC_NO, now, false, &autoshift_lastrecord);
         }
-        // For pressing another key while keyrepeating shifted autoshift.
-        del_weak_mods(MOD_BIT(KC_LEFT_SHIFT));
 
         switch (keycode) {
             case KC_ASTG:
                 autoshift_toggle();
-                return true;
+                break;
             case KC_ASON:
                 autoshift_enable();
-                return true;
+                break;
             case KC_ASOFF:
                 autoshift_disable();
-                return true;
+                break;
 
 #    ifndef AUTO_SHIFT_NO_SETUP
             case KC_ASUP:
                 autoshift_timeout += 5;
-                return true;
+                break;
             case KC_ASDN:
                 autoshift_timeout -= 5;
-                return true;
-
+                break;
             case KC_ASRP:
                 autoshift_timer_report();
-                return true;
+                break;
+#    endif
+        }
+        // If Retro Shift is disabled, possible custom actions shouldn't happen.
+        // clang-format off
+        if (IS_RETRO(keycode)
+#    if defined(RETRO_SHIFT) && !defined(NO_ACTION_TAPPING)
+            // Not tapped or #defines mean that rolls should use hold action.
+            && (
+                record->tap.count == 0
+#        ifdef RETRO_TAPPING_PER_KEY
+                || !get_retro_tapping(keycode, record)
+#        endif
+                || (record->tap.interrupted && (IS_LT(keycode)
+#        if defined(HOLD_ON_OTHER_KEY_PRESS) || defined(HOLD_ON_OTHER_KEY_PRESS_PER_KEY)
+#            ifdef HOLD_ON_OTHER_KEY_PRESS_PER_KEY
+                    ? get_hold_on_other_key_press(keycode, record)
+#            else
+                    ? true
+#            endif
+#        else
+                    ? false
+#        endif
+#        if defined(IGNORE_MOD_TAP_INTERRUPT) || defined(IGNORE_MOD_TAP_INTERRUPT_PER_KEY)
+#            ifdef IGNORE_MOD_TAP_INTERRUPT_PER_KEY
+                    : !get_ignore_mod_tap_interrupt(keycode, record)
+#            else
+                    : false
+#            endif
+#        else
+                    : true
+#        endif
+                ))
+            )
+#    endif
+        ) {
+            // clang-format on
+            autoshift_lastkey = KC_NO;
+            return true;
+        }
+    } else {
+        if (keycode == KC_LSFT) {
+            autoshift_flags.cancelling_lshift = false;
+        } else if (keycode == KC_RSFT) {
+            autoshift_flags.cancelling_rshift = false;
+        }
+        // Same as above (for pressed), additional checks are not needed because
+        // tap.count gets set to 0 in process_action
+        // clang-format off
+        else if (IS_RETRO(keycode)
+#    if defined(RETRO_SHIFT) && !defined(NO_ACTION_TAPPING)
+            && (
+                record->tap.count == 0
+#        ifdef RETRO_TAPPING_PER_KEY
+                || !get_retro_tapping(keycode, record)
+#        endif
+            )
 #    endif
+        ) {
+            // Fixes modifiers not being applied to rolls with AUTO_SHIFT_MODIFIERS set.
+#    if !defined(IGNORE_MOD_TAP_INTERRUPT) || defined(IGNORE_MOD_TAP_INTERRUPT_PER_KEY)
+            if (autoshift_flags.in_progress
+#        ifdef IGNORE_MOD_TAP_INTERRUPT_PER_KEY
+                && !get_ignore_mod_tap_interrupt(keycode, record)
+#        endif
+            ) {
+                autoshift_end(KC_NO, now, false, &autoshift_lastrecord);
+            }
+#    endif
+            // clang-format on
+            return true;
         }
     }
+
+    if (!autoshift_flags.enabled) {
+        return true;
+    }
     if (get_auto_shifted_key(keycode, record)) {
         if (record->event.pressed) {
             return autoshift_press(keycode, now, record);
         } else {
-            autoshift_end(keycode, now, false);
+            autoshift_end(keycode, now, false, record);
             return false;
         }
     }
+
+    // Prevent keyrepeating of older keys upon non-AS key event.
+    // Not commented at above returns but they serve the same function.
+    if (record->event.pressed) {
+        autoshift_lastkey = KC_NO;
+    }
     return true;
 }
 
-__attribute__((weak)) bool get_auto_shifted_key(uint16_t keycode, keyrecord_t *record) {
-    switch (keycode) {
-#    ifndef NO_AUTO_SHIFT_ALPHA
-        case KC_A ... KC_Z:
-#    endif
-#    ifndef NO_AUTO_SHIFT_NUMERIC
-        case KC_1 ... KC_0:
-#    endif
-#    ifndef NO_AUTO_SHIFT_SPECIAL
-        case KC_TAB:
-        case KC_MINUS ... KC_SLASH:
-        case KC_NONUS_BACKSLASH:
-#    endif
-            return true;
+#    if defined(RETRO_SHIFT) && !defined(NO_ACTION_TAPPING)
+// Called to record time before possible delays by action_tapping_process.
+void retroshift_poll_time(keyevent_t *event) {
+    last_retroshift_time = retroshift_time;
+    retroshift_time      = timer_read();
+}
+// Used to swap the times of Retro Shifted key and Auto Shift key that interrupted it.
+void retroshift_swap_times() {
+    if (last_retroshift_time != 0 && autoshift_flags.in_progress) {
+        uint16_t temp        = retroshift_time;
+        retroshift_time      = last_retroshift_time;
+        last_retroshift_time = temp;
     }
-    return false;
 }
+#    endif
 
 #endif
diff --git a/quantum/process_keycode/process_auto_shift.h b/quantum/process_keycode/process_auto_shift.h
index 00a9ab036..ac6a14374 100644
--- a/quantum/process_keycode/process_auto_shift.h
+++ b/quantum/process_keycode/process_auto_shift.h
@@ -22,13 +22,31 @@
 #    define AUTO_SHIFT_TIMEOUT 175
 #endif
 
+#define IS_LT(kc) ((kc) >= QK_LAYER_TAP && (kc) <= QK_LAYER_TAP_MAX)
+#define IS_MT(kc) ((kc) >= QK_MOD_TAP && (kc) <= QK_MOD_TAP_MAX)
+#define IS_RETRO(kc) (IS_MT(kc) || IS_LT(kc))
+#define DO_GET_AUTOSHIFT_TIMEOUT(keycode, record, ...) record
+// clang-format off
+#define AUTO_SHIFT_ALPHA KC_A ... KC_Z
+#define AUTO_SHIFT_NUMERIC KC_1 ... KC_0
+#define AUTO_SHIFT_SPECIAL          \
+             KC_TAB:                \
+        case KC_MINUS ... KC_SLASH: \
+        case KC_NONUS_BSLASH
+// clang-format on
+
 bool process_auto_shift(uint16_t keycode, keyrecord_t *record);
+void retroshift_poll_time(keyevent_t *event);
+void retroshift_swap_times(void);
 
 void     autoshift_enable(void);
 void     autoshift_disable(void);
 void     autoshift_toggle(void);
 bool     get_autoshift_state(void);
-uint16_t get_autoshift_timeout(void);
+uint16_t get_generic_autoshift_timeout(void);
+// clang-format off
+uint16_t (get_autoshift_timeout)(uint16_t keycode, keyrecord_t *record);
 void     set_autoshift_timeout(uint16_t timeout);
 void     autoshift_matrix_scan(void);
-bool     get_auto_shifted_key(uint16_t keycode, keyrecord_t *record);
+bool     get_custom_auto_shifted_key(uint16_t keycode, keyrecord_t *record);
+// clang-format on