Heatmap incorrect matrix effect workaround (qmk#16315)

docs/feature_rgb_matrix.md

@@ -657,18 +657,19 @@ You can enable a single effect by defining `ENABLE_[EFFECT_NAME]` in your `confi
 
 ### RGB Matrix Effect Typing Heatmap :id=rgb-matrix-effect-typing-heatmap
 
-This effect will color the RGB matrix according to a heatmap of recently pressed
-keys. Whenever a key is pressed its "temperature" increases as well as that of
-its neighboring keys. The temperature of each key is then decreased
-automatically every 25 milliseconds by default.
+This effect will color the RGB matrix according to a heatmap of recently pressed keys. Whenever a key is pressed its "temperature" increases as well as that of its neighboring keys. The temperature of each key is then decreased automatically every 25 milliseconds by default.
 
-In order to change the delay of temperature decrease define
-`RGB_MATRIX_TYPING_HEATMAP_DECREASE_DELAY_MS`:
+In order to change the delay of temperature decrease define `RGB_MATRIX_TYPING_HEATMAP_DECREASE_DELAY_MS`:
 
 ```c
 #define RGB_MATRIX_TYPING_HEATMAP_DECREASE_DELAY_MS 50
 ```
 
+Heatmap effect may not light up the correct adjacent LEDs for certain key matrix layout such as split keyboards. The following define will limit the effect to pressed keys only:
+```c
+#define RGB_MATRIX_TYPING_HEATMAP_SLIM
+```
+
 ## Custom RGB Matrix Effects :id=custom-rgb-matrix-effects
 
 By setting `RGB_MATRIX_CUSTOM_USER = yes` in `rules.mk`, new effects can be defined directly from your keymap or userspace, without having to edit any QMK core files. To declare new effects, create a `rgb_matrix_user.inc` file in the user keymap directory or userspace folder.

quantum/rgb_matrix/animations/typing_heatmap_anim.h

@@ -8,6 +8,10 @@ RGB_MATRIX_EFFECT(TYPING_HEATMAP)
 #        endif
 
 void process_rgb_matrix_typing_heatmap(uint8_t row, uint8_t col) {
+#        ifdef RGB_MATRIX_TYPING_HEATMAP_SLIM
+    // Limit effect to pressed keys
+    g_rgb_frame_buffer[row][col] = qadd8(g_rgb_frame_buffer[row][col], 32);
+#        else
     uint8_t m_row = row - 1;
     uint8_t p_row = row + 1;
     uint8_t m_col = col - 1;
@@ -28,6 +32,7 @@ void process_rgb_matrix_typing_heatmap(uint8_t row, uint8_t col) {
         g_rgb_frame_buffer[m_row][col] = qadd8(g_rgb_frame_buffer[m_row][col], 16);
         if (p_col < MATRIX_COLS) g_rgb_frame_buffer[m_row][p_col] = qadd8(g_rgb_frame_buffer[m_row][p_col], 13);
     }
+#        endif
 }
 
 // A timer to track the last time we decremented all heatmap values.