Rework bcat Crkbd and Lily58 modifier keys (#9692)

* Re-enable mouse keys to fix Chrome OS media keys

I'm not sure if there's a bug in Chrome OS, QMK, or both, but
EXTRAKEY_ENABLE isn't sufficient for media keys to work on Chrome OS.
Instead, MOUSEKEY_ENABLE is also required.

* Remove unnecessary SPLIT_USB_DETECT for Lily58

I've since swapped my Lily58 back to Elite-C v2 controllers with working
VBUS detection.

* Move Crkbd Esc and Ctrl keys; add some shortcuts

* Move MC_ALTT to userspace for cross-board support

* Sync Lily58 keymap with Crkbd

* Fix typos

3 files changed, 49 insertions, 3 deletions

diff --git a/users/bcat/bcat.c b/users/bcat/bcat.c
index 2b250c10f..397d565da 100644
--- a/users/bcat/bcat.c
+++ b/users/bcat/bcat.c
@@ -1,6 +1,43 @@
-#include "quantum.h"
+#include "bcat.h"
 
 #if defined(RGBLIGHT_ENABLE)
 /* Adjust RGB static hue ranges for shorter gradients than default. */
 const uint8_t RGBLED_GRADIENT_RANGES[] PROGMEM = {255, 127, 63, 31, 15};
 #endif
+
+static int8_t alt_tab_layer = -1;
+
+__attribute__((weak)) bool process_record_keymap(uint16_t keycode, keyrecord_t *record) { return true; }
+
+bool process_record_user(uint16_t keycode, keyrecord_t *record) {
+    if (!process_record_keymap(keycode, record)) {
+        return false;
+    }
+    switch (keycode) {
+        /* Alt+Tab that holds Alt until current layer is released: */
+        case MC_ALTT:
+            if (record->event.pressed) {
+                if (alt_tab_layer < 0) {
+                    alt_tab_layer = layer_switch_get_layer(record->event.key);
+                    register_code(KC_LALT);
+                }
+                register_code(KC_TAB);
+            } else {
+                unregister_code(KC_TAB);
+            }
+            return false;
+        default:
+            return true;
+    }
+}
+
+__attribute__((weak)) layer_state_t layer_state_set_keymap(layer_state_t state) { return state; }
+
+layer_state_t layer_state_set_user(layer_state_t state) {
+    state = layer_state_set_keymap(state);
+    if (alt_tab_layer >= 0 && !layer_state_cmp(state, alt_tab_layer)) {
+        unregister_code(KC_LALT);
+        alt_tab_layer = -1;
+    }
+    return state;
+}
diff --git a/users/bcat/bcat.h b/users/bcat/bcat.h
new file mode 100644
index 000000000..1ea05e5fa
--- /dev/null
+++ b/users/bcat/bcat.h
@@ -0,0 +1,8 @@
+#pragma once
+
+#include "quantum.h"
+
+enum user_keycodes {
+    MC_ALTT = SAFE_RANGE,
+    KEYMAP_SAFE_RANGE,
+};
diff --git a/users/bcat/rules.mk b/users/bcat/rules.mk
index f3c6f9ab2..f979c703f 100644
--- a/users/bcat/rules.mk
+++ b/users/bcat/rules.mk
@@ -3,8 +3,10 @@ SRC += bcat.c
 # Enable Bootmagic Lite to consistently reset to bootloader and clear EEPROM.
 BOOTMAGIC_ENABLE = lite
 
-# Enable media keys on all keyboards.
+# Enable media keys on all keyboards. (Even though I don't use mouse keys, they
+# seem to be required for media keys to register on Chrome OS.)
 EXTRAKEY_ENABLE = yes
+MOUSEKEY_ENABLE = yes
 
 # Enable link-time optimization to reduce binary size.
 LINK_TIME_OPTIMIZATION_ENABLE = yes
@@ -12,7 +14,6 @@ LINK_TIME_OPTIMIZATION_ENABLE = yes
 # Disable unused build options on all keyboards.
 COMMAND_ENABLE = no
 CONSOLE_ENABLE = no
-MOUSEKEY_ENABLE = no
 NKRO_ENABLE = no
 TERMINAL_ENABLE = no