[Keymap] New custom 'super alt' keymap for the Drop ALT (#9385)

* New custom 'super alt' keymap for the Drop ALT

* Improvements to 'super alt' keymap based on PR feedback

* Fix flickering LED caps lock bug

* Code cleanup from PR feedback

* Minor keymap layout cleanup

keyboards/massdrop/alt/keymaps/ewersp/README.md

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+## Greetings, Traveler!
+
+This layout is for the [Massdrop ALT Keyboard](https://drop.com/buy/massdrop-alt-mechanical-keyboard) and targets Windows machines.
+
+This keymap is very similar to the default keymap for the Drop ALT, but it overloads the left alt key to behave like an additional 'Fn' key while keeping the functionality of the alt key intact.
+
+### For example:
+```
+LeftAlt + 1...N      -> F1...FN
+LeftAlt + L/R Arrows -> Home/End
+LeftAlt + Home       -> End
+LeftAlt + Backspace  -> Delete
+LeftAlt + Esc        -> Backtick (`)
+```
+
+The novel part of this keymap is that it **preserves the default functionality** of the left alt key, so ```'alt + tab', 'ctrl + alt + del', 'alt + f4'```, etc. all work as expected without using any janky timers.
+
+Everything also works with other modifier keys, so ```'alt + L/R' and 'alt + shift + L/R'``` work great for text manipulation to select lines.
+
+This **'super alt'** keymap will feel very familiar for anyone **coming from macOS** since the Windows alt key is in the same position as CMD on macOS keyboards.
+
+And finally, the original alt key functionality can be toggled at any time by pressing **```'Fn + period'```** so even if you don't like it, you can turn it off.
+
+This keymap was based on the 'default' Drop ALT keymap (and _not_ 'default_md') which means it supports all the fancy QMK RGB patterns, and isn't limited to the stock options that come with the board.
+
+It's also worth noting that this keymap fixes an outstanding issue where the caps lock LED flickers when the RGB mode is keylight or underglow. I could not find a general purpose fix, but the one included here works quite well.
+
+If you have any questions, feel free to reach out to me at: ewersp [at] gmail [dot] com.
+
+Enjoy! **<3**

keyboards/massdrop/alt/keymaps/ewersp/config.h

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+#pragma once
+
+#define FORCE_NKRO

keyboards/massdrop/alt/keymaps/ewersp/keymap.c

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+#include QMK_KEYBOARD_H
+
+enum alt_keycodes {
+    U_T_AUTO = SAFE_RANGE, //USB Extra Port Toggle Auto Detect / Always Active
+    U_T_AGCR,              //USB Toggle Automatic GCR control
+    DBG_TOG,               //DEBUG Toggle On / Off
+    DBG_MTRX,              //DEBUG Toggle Matrix Prints
+    DBG_KBD,               //DEBUG Toggle Keyboard Prints
+    DBG_MOU,               //DEBUG Toggle Mouse Prints
+    MD_BOOT,               //Restart into bootloader after hold timeout
+    ALT_DEL,               //Added to map left alt + backspace to delete
+};
+
+// Friendly layer names
+enum alt_layers {
+    DEF = 0,
+    ALT,
+    FUNC,
+    SUPR
+};
+
+const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
+    [DEF] = LAYOUT_65_ansi_blocker(
+        KC_ESC,  KC_1,    KC_2,    KC_3,    KC_4,    KC_5,    KC_6,    KC_7,    KC_8,    KC_9,    KC_0,    KC_MINS, KC_EQL,  KC_BSPC, KC_DEL,
+        KC_TAB,  KC_Q,    KC_W,    KC_E,    KC_R,    KC_T,    KC_Y,    KC_U,    KC_I,    KC_O,    KC_P,    KC_LBRC, KC_RBRC, KC_BSLS, KC_HOME,
+        KC_CAPS, KC_A,    KC_S,    KC_D,    KC_F,    KC_G,    KC_H,    KC_J,    KC_K,    KC_L,    KC_SCLN, KC_QUOT,          KC_ENT,  KC_PGUP,
+        KC_LSFT, KC_Z,    KC_X,    KC_C,    KC_V,    KC_B,    KC_N,    KC_M,    KC_COMM, KC_DOT,  KC_SLSH, KC_RSFT,          KC_UP,   KC_PGDN,
+        KC_LCTL, KC_LGUI, MO(SUPR),                           KC_SPC,                             KC_RALT, MO(FUNC),KC_LEFT, KC_DOWN, KC_RGHT
+    ),
+    [ALT] = LAYOUT_65_ansi_blocker(
+        _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______,
+        _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______,
+        _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______,          _______, _______,
+        _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______,          _______, _______,
+        _______, _______, KC_LALT,                            _______,                            _______, _______, _______, _______, _______
+    ),
+    [FUNC] = LAYOUT_65_ansi_blocker(
+        KC_GRV,  KC_F1,   KC_F2,   KC_F3,   KC_F4,   KC_F5,   KC_F6,   KC_F7,   KC_F8,   KC_F9,   KC_F10,  KC_F11,  KC_F12,  _______, KC_MUTE,
+        _______, RGB_SPD, RGB_VAI, RGB_SPI, RGB_HUI, RGB_SAI, _______, U_T_AUTO,U_T_AGCR,_______, KC_PSCR, KC_SLCK, KC_PAUS, _______, KC_END,
+        _______, RGB_RMOD,RGB_VAD, RGB_MOD, RGB_HUD, RGB_SAD, _______, _______, _______, _______, _______, _______,          _______, KC_VOLU,
+        _______, RGB_TOG, _______, _______, _______, MD_BOOT, NK_TOGG, DBG_TOG, _______, TG(ALT), _______, _______,          KC_PGUP, KC_VOLD,
+        _______, _______, KC_LALT,                            _______,                            _______, _______, KC_HOME, KC_PGDN, KC_END
+    ),
+    [SUPR] = LAYOUT_65_ansi_blocker(
+        KC_GRV,  KC_F1,   KC_F2,   KC_F3,   KC_F4,   KC_F5,   KC_F6,   KC_F7,   KC_F8,   KC_F9,   KC_F10,  KC_F11,  KC_F12,  ALT_DEL, _______,
+        _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, KC_END,
+        _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______,          _______, _______,
+        _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______,          _______, _______,
+        _______, _______, _______,                            _______,                            _______, _______, KC_HOME, _______, KC_END
+    )
+};
+
+// If the super alt layer is the active layer
+bool super_alt_layer_active = false;
+
+// If we need to unregister alt when leaving the super alt layer
+bool need_to_unregister_alt = false;
+
+#define MODS_SHIFT  (get_mods() & MOD_BIT(KC_LSHIFT) || get_mods() & MOD_BIT(KC_RSHIFT))
+#define MODS_CTRL  (get_mods() & MOD_BIT(KC_LCTL) || get_mods() & MOD_BIT(KC_RCTRL))
+#define MODS_ALT  (get_mods() & MOD_BIT(KC_LALT) || get_mods() & MOD_BIT(KC_RALT))
+
+// Taken from 'g_led_config' in config_led.c
+#define CAPS_LOCK_LED_ID 30
+
+// This runs every matrix scan (every 'frame')
+void rgb_matrix_indicators_user(void) {
+    led_flags_t flags = rgb_matrix_get_flags();
+
+    // If we're in either keylight or underglow modes (but not both simultaneously)
+    if (HAS_FLAGS(flags, LED_FLAG_KEYLIGHT) != HAS_FLAGS(flags, LED_FLAG_UNDERGLOW)) {
+
+        // This fixes a bug where the caps lock LED flickers when toggled in either keylight or underglow modes
+        if (host_keyboard_leds() & (1 << USB_LED_CAPS_LOCK)) {
+            rgb_matrix_set_color(CAPS_LOCK_LED_ID, RGB_WHITE);
+        } else {
+            rgb_matrix_set_color(CAPS_LOCK_LED_ID, 0, 0, 0);
+        }
+    }
+}
+
+// This runs code every time that the layers get changed
+layer_state_t layer_state_set_user(layer_state_t state) {
+    switch (get_highest_layer(state)) {
+        case DEF:
+            // When returning to the default layer, check if we need to unregister the left alt key
+            if (super_alt_layer_active && need_to_unregister_alt) {
+                unregister_code(KC_LALT);
+            }
+
+            super_alt_layer_active = false;
+            need_to_unregister_alt = false;
+            break;
+        case SUPR:
+            super_alt_layer_active = true;
+            break;
+    }
+    return state;
+}
+
+bool process_record_user(uint16_t keycode, keyrecord_t *record) {
+    static uint32_t key_timer;
+
+    switch (keycode) {
+        // These are the keys we want to 'fall though' and behave as usual when pressed with the alt modifier
+        case KC_A ... KC_Z:
+        case KC_TAB:
+        case KC_DEL:
+        case KC_UP:
+        case KC_DOWN:
+        case MO(FUNC):
+            if (super_alt_layer_active && record->event.pressed) {
+                // Only activate the alt modifier for the first key press
+                if ((get_mods() & MOD_BIT(KC_LALT)) == false) {
+                    register_code(KC_LALT);
+                    need_to_unregister_alt = true;
+                }
+            }
+            // We still want to process the keycode normally
+            return true;
+        case ALT_DEL:
+            if (record->event.pressed) {
+                register_code(KC_DEL);
+            } else {
+                unregister_code(KC_DEL);
+            }
+            return false;
+        case U_T_AUTO:
+            if (record->event.pressed && MODS_SHIFT && MODS_CTRL) {
+                TOGGLE_FLAG_AND_PRINT(usb_extra_manual, "USB extra port manual mode");
+            }
+            return false;
+        case U_T_AGCR:
+            if (record->event.pressed && MODS_SHIFT && MODS_CTRL) {
+                TOGGLE_FLAG_AND_PRINT(usb_gcr_auto, "USB GCR auto mode");
+            }
+            return false;
+        case DBG_TOG:
+            if (record->event.pressed) {
+                TOGGLE_FLAG_AND_PRINT(debug_enable, "Debug mode");
+            }
+            return false;
+        case DBG_MTRX:
+            if (record->event.pressed) {
+                TOGGLE_FLAG_AND_PRINT(debug_matrix, "Debug matrix");
+            }
+            return false;
+        case DBG_KBD:
+            if (record->event.pressed) {
+                TOGGLE_FLAG_AND_PRINT(debug_keyboard, "Debug keyboard");
+            }
+            return false;
+        case DBG_MOU:
+            if (record->event.pressed) {
+                TOGGLE_FLAG_AND_PRINT(debug_mouse, "Debug mouse");
+            }
+            return false;
+        case MD_BOOT:
+            if (record->event.pressed) {
+                key_timer = timer_read32();
+            } else {
+                if (timer_elapsed32(key_timer) >= 500) {
+                    reset_keyboard();
+                }
+            }
+            return false;
+        case RGB_TOG:
+            if (record->event.pressed) {
+              switch (rgb_matrix_get_flags()) {
+                case LED_FLAG_ALL: {
+                    rgb_matrix_set_flags(LED_FLAG_KEYLIGHT);
+                    rgb_matrix_set_color_all(0, 0, 0);
+                  }
+                  break;
+                case LED_FLAG_KEYLIGHT: {
+                    rgb_matrix_set_flags(LED_FLAG_UNDERGLOW);
+                    rgb_matrix_set_color_all(0, 0, 0);
+                  }
+                  break;
+                case LED_FLAG_UNDERGLOW: {
+                    rgb_matrix_set_flags(LED_FLAG_NONE);
+                    rgb_matrix_disable_noeeprom();
+                  }
+                  break;
+                default: {
+                    rgb_matrix_set_flags(LED_FLAG_ALL);
+                    rgb_matrix_enable_noeeprom();
+                  }
+                  break;
+              }
+            }
+            return false;
+        default:
+            return true; // Process all other keycodes normally
+    }
+}