fix(live-preview): suppress md viewer ↔ CM scroll feedback loops

Selecting/clicking/editing in the md viewer caused the iframe to scroll
itself to a stale line, and undo/redo plus backspace caused visible
scroll jumps. Root cause was missing suppression on both sides of the
sync bridge:

- The iframe side: viewer-initiated events that can cause CM to scroll
  (selection sync, cursor-line sync, click-to-focus) didn't mark the
  iframe as the scroll origin, so CM's resulting scroll-handler echo
  came back through MDVIEWR_SCROLL_TO_LINE and re-scrolled the iframe.
  Centralize the flag in sendToParent for all such events, and drop the
  editMode-only requirement from the guard so design mode is covered too.

- The CM side: cm.replaceRange (in _applyDiffToEditor), cm.scrollTo (in
  _scrollCMToLine), and cm.undo()/cm.redo() can each trigger an async
  CM scroll that echoes back to the iframe. Wrap each in
  _scrollSyncFromIframe = true with a 200ms clear so CM's scroll handler
  ignores them.

Author: abose

src-mdviewer/src/bridge.js

@@ -15,6 +15,7 @@ let _lastReceivedSyncId = -1;
 let _suppressContentChange = false;
 let _scrollFromCM = false;
 let _scrollFromViewer = false;
+let _scrollFromViewerTimer = null;
 let _suppressScrollToLine = false;
 let _baseURL = "";
 let _cursorPosBeforeEdit = null; // cursor position before current edit batch
@@ -513,6 +514,11 @@ export function initBridge() {
                 }
                 if (bestEl) {
                     const sourceLine = parseInt(bestEl.getAttribute("data-source-line"), 10);
+                    // Mark the viewer as the scroll origin so CM's echo back
+                    // through handleScrollToLine is suppressed.
+                    _scrollFromViewer = true;
+                    if (_scrollFromViewerTimer) clearTimeout(_scrollFromViewerTimer);
+                    _scrollFromViewerTimer = setTimeout(() => { _scrollFromViewer = false; }, 400);
                     sendToParent("mdviewrScrollSync", { sourceLine, fromScroll: true });
                 }
             });
@@ -1057,9 +1063,9 @@ function handleScrollToLine(data) {
     // Suppress during file switch — doc cache restores the correct scroll
     if (_suppressScrollToLine) return;
 
-    // In edit mode, ignore scroll-based sync that originated from the viewer
-    // itself (feedback loop: viewer click → CM scroll → scroll sync back).
-    if (fromScroll && getState().editMode && _scrollFromViewer) return;
+    // Ignore scroll-based sync that originated from the viewer itself
+    // (feedback loop: viewer scroll/click → CM scroll → scroll sync back).
+    if (fromScroll && _scrollFromViewer) return;
 
     const viewer = document.getElementById("viewer-content");
     if (!viewer) return;
@@ -1401,8 +1407,23 @@ function _sendSelectionToParent() {
     }, 200);
 }
 
+// Events the iframe initiates that can cause CM to scroll. We mark the iframe
+// as the scroll origin so CM's resulting scroll-handler echo (forwarded back as
+// MDVIEWR_SCROLL_TO_LINE) gets suppressed by the guard in handleScrollToLine.
+const _viewerInitiatedScrollEvents = new Set([
+    "mdviewrScrollSync",
+    "mdviewrCursorLine",
+    "mdviewrSelectionSync",
+    "embeddedIframeFocusEditor"
+]);
+
 function sendToParent(eventName, payload) {
     if (!window.parent || window.parent === window) return;
+    if (_viewerInitiatedScrollEvents.has(eventName)) {
+        _scrollFromViewer = true;
+        if (_scrollFromViewerTimer) clearTimeout(_scrollFromViewerTimer);
+        _scrollFromViewerTimer = setTimeout(() => { _scrollFromViewer = false; }, 400);
+    }
     window.parent.postMessage({
         type: "MDVIEWR_EVENT",
         eventName,

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

@@ -578,8 +578,14 @@ define(function (require, exports, module) {
         const replacement = newText.substring(prefixLen, newSuffix);
 
         _syncingFromIframe = true;
+        // Suppress CM's scroll-handler echo for a brief window: replaceRange
+        // can trigger an asynchronous CM scroll (e.g. when content shrinks or
+        // cursor is auto-scrolled into view) which would otherwise feed back to
+        // the iframe and snap it to a stale "first visible line".
+        _scrollSyncFromIframe = true;
         cm.replaceRange(replacement, fromPos, toPos, "+mdviewr");
         _syncingFromIframe = false;
+        setTimeout(function () { _scrollSyncFromIframe = false; }, 200);
     }
 
     function _onIframeContentChanged(data) {
@@ -648,7 +654,12 @@ define(function (require, exports, module) {
                 _cursorRedoStack.push(pos);
                 _pendingCursorPos = pos;
             }
+            // cm.undo() can move the cursor and scroll it into view, which
+            // triggers CM's scroll handler and echoes back to the iframe as
+            // a fromScroll sync — causing visible scroll jumps. Suppress.
+            _scrollSyncFromIframe = true;
             cm.undo();
+            setTimeout(function () { _scrollSyncFromIframe = false; }, 200);
         }
     }
 
@@ -663,7 +674,9 @@ define(function (require, exports, module) {
                 _cursorUndoStack.push(pos);
                 _pendingCursorPos = pos;
             }
+            _scrollSyncFromIframe = true;
             cm.redo();
+            setTimeout(function () { _scrollSyncFromIframe = false; }, 200);
         }
     }
 
@@ -862,7 +875,11 @@ define(function (require, exports, module) {
 
         if (lineTop < viewTop || lineBottom > viewBottom) {
             const targetScrollTop = lineTop - (scrollInfo.clientHeight / 2);
+            // Suppress CM's scroll-handler echo while we scroll programmatically
+            // — otherwise the echo loops back to the iframe and re-scrolls it.
+            _scrollSyncFromIframe = true;
             cm.scrollTo(null, targetScrollTop);
+            setTimeout(function () { _scrollSyncFromIframe = false; }, 200);
         }
 
         // Brief flash on the CM line to show cursor sync feedback