perf(mdviewer): instant viewer→CM cursor highlight via fast path

Add _sendCursorLineToParent() that sends just the source line on
selectionchange without debounce. MarkdownSync handles the new
mdviewrCursorLine message by immediately updating the CM line
highlight without scrolling. Eliminates the visible delay when
moving cursor in the viewer.

Author: abose

src-mdviewer/src/bridge.js

@@ -472,6 +472,9 @@ export function initBridge() {
         if (!_cursorPosDirty) {
             _cursorPosBeforeEdit = _getCursorPosition();
         }
+        // Fast path: send just the source line for instant CM highlight
+        _sendCursorLineToParent();
+        // Full selection sync (debounced)
         _sendSelectionToParent();
     });
 
@@ -1065,6 +1068,19 @@ function handleHighlightSelection(data) {
     }
 }
 
+// Fast path: send just the cursor's source line for instant CM highlight (no debounce)
+function _sendCursorLineToParent() {
+    const selection = window.getSelection();
+    if (!selection || !selection.rangeCount) return;
+    const anchorNode = selection.anchorNode;
+    const el = anchorNode && (anchorNode.nodeType === Node.ELEMENT_NODE
+        ? anchorNode : anchorNode.parentElement);
+    const sourceLine = _getSourceLineFromElement(el);
+    if (sourceLine != null) {
+        sendToParent("mdviewrCursorLine", { sourceLine });
+    }
+}
+
 let _selectionSendTimer = null;
 function _sendSelectionToParent() {
     clearTimeout(_selectionSendTimer);

src/extensionsIntegrated/Phoenix-live-preview/MarkdownSync.js

@@ -153,6 +153,15 @@ define(function (require, exports, module) {
                     }
                 }
                 break;
+            case "mdviewrCursorLine":
+                // Fast path: just update CM line highlight, no scroll
+                if (_cursorSyncEnabled && data.sourceLine != null) {
+                    const cm = _getCM();
+                    if (cm) {
+                        _flashCMLine(cm, Math.max(0, data.sourceLine - 1));
+                    }
+                }
+                break;
             case "mdviewrSelectionSync":
                 if (_cursorSyncEnabled) {
                     _handleSelectionFromIframe(data);