fix(live-preview): keep CM edits from being undone by stale md viewer reports

The md viewer flushes its own debounced edit when the same file is
switched to again, and that report could reach Phoenix just after the
next CM edit. MarkdownSync applied it, silently undoing the CM change,
and the viewer then synced back the reverted text. This made the
"editing code block content in CM sync to viewer" test flaky on CI.

While a CM change is still waiting to be sent to the iframe, the iframe
has not seen it, so its content reports are stale: ignore them and let
the pending update bring the iframe up to date.

Author: abose

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

@@ -272,6 +272,7 @@ define(function (require, exports, module) {
             }
             clearTimeout(_debounceTimer);
             _debounceTimer = setTimeout(function () {
+                _debounceTimer = null;
                 _sendUpdate(_lastChangeOrigin);
                 _lastChangeOrigin = null;
             }, DEBOUNCE_TO_IFRAME_MS);
@@ -359,6 +360,7 @@ define(function (require, exports, module) {
         }
 
         clearTimeout(_debounceTimer);
+        _debounceTimer = null;
         clearTimeout(_scrollSyncTimer);
         clearTimeout(_selectionSyncTimer);
 
@@ -672,6 +674,15 @@ define(function (require, exports, module) {
             return;
         }
 
+        // A CM change is still waiting to go to the iframe, so the iframe made this
+        // before it had that change: applying it would undo the change. The pending
+        // update brings the iframe up to date instead. (e.g. the iframe flushes its
+        // own debounced edit when the same file is switched to again, and that can
+        // land just after the user's next CM edit.)
+        if (_debounceTimer) {
+            return;
+        }
+
         const markdown = data.markdown;
         const remoteSyncId = data._syncId;