Fix cross-half reactive RGB splash on BCORNE split.

Mirror master matrix state and sync remote keypresses to rgb_matrix so splash propagates from the left half onto the right, and close the LED coordinate gap at the split seam.

Co-authored-by: Cursor <cursoragent@cursor.com>

corne/firmware/matrix.c

@@ -26,6 +26,10 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #    include "split_common/split_util.h"
 #    include "split_common/transactions.h"
 
+#ifdef RGB_MATRIX_ENABLE
+#    include "rgb_matrix.h"
+#endif
+
 #    define ROWS_PER_HAND (MATRIX_ROWS / 2)
 
 
@@ -229,6 +233,45 @@ bool is_keyboard_master_impl(void) {
 
 static bool  last_connected              = false;
 uint32_t counter1 = 1;
+
+#ifdef RGB_MATRIX_ENABLE
+static void rgb_matrix_sync_remote_half(matrix_row_t remote_prev[ROWS_PER_HAND], uint8_t remote_offset) {
+    for (uint8_t r = 0; r < ROWS_PER_HAND; r++) {
+        const uint8_t      row     = remote_offset + r;
+        const matrix_row_t current = matrix[row];
+        const matrix_row_t delta   = current ^ remote_prev[r];
+
+        if (!delta) {
+            continue;
+        }
+
+        matrix_row_t col_mask = 1;
+        for (uint8_t col = 0; col < MATRIX_COLS; col++, col_mask <<= 1) {
+            if (delta & col_mask) {
+                rgb_matrix_handle_key_event(row, col, current & col_mask);
+            }
+        }
+
+        remote_prev[r] = current;
+    }
+}
+#endif
+
+void matrix_scan_kb(void) {
+#ifdef RGB_MATRIX_ENABLE
+    static matrix_row_t master_remote_prev[ROWS_PER_HAND] = {0};
+    static matrix_row_t slave_mirror_prev[ROWS_PER_HAND]  = {0};
+
+    if (is_keyboard_master()) {
+        rgb_matrix_sync_remote_half(master_remote_prev, thatHand);
+    } else {
+        rgb_matrix_sync_remote_half(slave_mirror_prev, thatHand);
+    }
+#endif
+
+    matrix_scan_user();
+}
+
 bool matrix_post_scan_qf(void) {
     counter1++;
     bool changed = false;
@@ -248,8 +291,6 @@ bool matrix_post_scan_qf(void) {
         }
 
         if (changed) memcpy(matrix + thatHand, slave_matrix, sizeof(slave_matrix));
-
-        matrix_scan_kb();
     } else {
          if(counter1 % 15000 ==0){
             // extern  bool usb_bus_detected(void);
@@ -262,11 +303,20 @@ bool matrix_post_scan_qf(void) {
                  NVIC_SystemReset();
             }
         }
+        matrix_row_t remote_before[ROWS_PER_HAND];
+        memcpy(remote_before, matrix + thatHand, sizeof(remote_before));
+
         transport_slave(matrix + thatHand, matrix + thisHand);
 
+        if (memcmp(remote_before, matrix + thatHand, sizeof(remote_before)) != 0) {
+            changed = true;
+        }
+
         matrix_slave_scan_kb();
     }
 
+    matrix_scan_kb();
+
     return changed;
 }