Fix up process_leader to be a bit more optimized (#4662)

* Fix up process_leader to be a bit more optimized

* Process dual function keys better

* Make leader start a callable function

* Fix per key timer call location

* Add escape if already leading

* Return false for KC_LEAD

* Add documentation

4 files changed, 40 insertions, 21 deletions

diff --git a/docs/config_options.md b/docs/config_options.md
index 879761a40..4bbc5debd 100644
--- a/docs/config_options.md
+++ b/docs/config_options.md
@@ -146,6 +146,8 @@ If you define these options you will enable the associated feature, which may in
     * If you're having issues finishing the sequence before it times out, you may need to increase the timeout setting. Or you may want to enable the `LEADER_PER_KEY_TIMING` option, which resets the timeout after each key is tapped. 
 * `#define LEADER_PER_KEY_TIMING`
   * sets the timer for leader key chords to run on each key press rather than overall
+* `#define LEADER_KEY_STRICT_KEY_PROCESSING`
+  * Disables keycode filtering for Mod-Tap and Layer-Tap keycodes. Eg, if you enable this, you would need to specify `MT(MOD_CTL, KC_A)` if you want to use `KC_A`.
 * `#define ONESHOT_TIMEOUT 300`
   * how long before oneshot times out
 * `#define ONESHOT_TAP_TOGGLE 2`
diff --git a/docs/feature_leader_key.md b/docs/feature_leader_key.md
index 168a1fc1c..82cf78901 100644
--- a/docs/feature_leader_key.md
+++ b/docs/feature_leader_key.md
@@ -72,6 +72,12 @@ SEQ_THREE_KEYS(KC_C, KC_C, KC_C) {
 }
 ```
 
+## Strict Key Processing
+
+By default, the Leader Key feature will filter the keycode out of [`Mod-Tap`](feature_advanced_keycodes.md#mod-tap) and [`Layer Tap`](feature_advanced_keycodes.md#switching-and-toggling-layers) functions when checking for the Leader sequences. That means if you're using `LT(3, KC_A)`, it will pick this up as `KC_A` for the sequence, rather than `LT(3, KC_A)`, giving a more expected behavior for newer users.
+
+While, this may be fine for most, if you want to specify the whole keycode (eg, `LT(3, KC_A)` from the example above) in the sequence, you can enable this by added `#define LEADER_KEY_STRICT_KEY_PROCESSING` to your `config.h` file.  This well then disable the filtering, and you'll need to specify the whole keycode.
+
 ## Customization 
 
 The Leader Key feature has some additional customization to how the Leader Key feature works.  It has two functions that can be called at certain parts of the process.  Namely `leader_start()` and `leader_end()`.
diff --git a/quantum/process_keycode/process_leader.c b/quantum/process_keycode/process_leader.c
index b32fc1db6..57fccdc7e 100644
--- a/quantum/process_keycode/process_leader.c
+++ b/quantum/process_keycode/process_leader.c
@@ -35,31 +35,42 @@ uint16_t leader_time = 0;
 uint16_t leader_sequence[5] = {0, 0, 0, 0, 0};
 uint8_t leader_sequence_size = 0;
 
+void qk_leader_start(void) {
+  if (leading) { return; }
+  leader_start();
+  leading = true;
+  leader_time = timer_read();
+  leader_sequence_size = 0;
+  leader_sequence[0] = 0;
+  leader_sequence[1] = 0;
+  leader_sequence[2] = 0;
+  leader_sequence[3] = 0;
+  leader_sequence[4] = 0;
+}
+
 bool process_leader(uint16_t keycode, keyrecord_t *record) {
   // Leader key set-up
   if (record->event.pressed) {
+    if (leading) {
+      if (timer_elapsed(leader_time) < LEADER_TIMEOUT) {
+#ifndef LEADER_KEY_STRICT_KEY_PROCESSING
+        if ((keycode >= QK_MOD_TAP && keycode <= QK_MOD_TAP_MAX) || (keycode >= QK_LAYER_TAP && keycode <= QK_LAYER_TAP_MAX)) {
+          keycode = keycode & 0xFF;
+        }
+#endif // LEADER_KEY_STRICT_KEY_PROCESSING
+        leader_sequence[leader_sequence_size] = keycode;
+        leader_sequence_size++;
 #ifdef LEADER_PER_KEY_TIMING
-    leader_time = timer_read();
+        leader_time = timer_read();
 #endif
-    if (!leading && keycode == KC_LEAD) {
-      leader_start();
-      leading = true;
-#ifndef LEADER_PER_KEY_TIMING
-      leader_time = timer_read();
-#endif
-      leader_time = timer_read();
-      leader_sequence_size = 0;
-      leader_sequence[0] = 0;
-      leader_sequence[1] = 0;
-      leader_sequence[2] = 0;
-      leader_sequence[3] = 0;
-      leader_sequence[4] = 0;
-      return false;
-    }
-    if (leading && timer_elapsed(leader_time) < LEADER_TIMEOUT) {
-      leader_sequence[leader_sequence_size] = keycode;
-      leader_sequence_size++;
-      return false;
+        return false;
+      }
+    } else {
+      if (keycode == KC_LEAD) {
+        qk_leader_start();
+        return false;
+      }
+      break;
     }
   }
   return true;
diff --git a/quantum/process_keycode/process_leader.h b/quantum/process_keycode/process_leader.h
index 59c3eed1b..15bccc3f6 100644
--- a/quantum/process_keycode/process_leader.h
+++ b/quantum/process_keycode/process_leader.h
@@ -24,7 +24,7 @@ bool process_leader(uint16_t keycode, keyrecord_t *record);
 
 void leader_start(void);
 void leader_end(void);
-
+void qk_leader_start(void);
 
 #define SEQ_ONE_KEY(key) if (leader_sequence[0] == (key) && leader_sequence[1] == 0 && leader_sequence[2] == 0 && leader_sequence[3] == 0 && leader_sequence[4] == 0)
 #define SEQ_TWO_KEYS(key1, key2) if (leader_sequence[0] == (key1) && leader_sequence[1] == (key2) && leader_sequence[2] == 0 && leader_sequence[3] == 0 && leader_sequence[4] == 0)