fix(mdviewer): scroll position lost on panel reopen, theme reflow guard

- Fix scroll position destroyed when panel is hidden then reopened:
  saveActiveScrollPos now skips saving when viewer is hidden (scrollTop=0
  from hidden element would overwrite the correct cached value)
- Stricter theme skip: check both data-theme attr and colorScheme style
- Increase panel reopen scroll test timeout for CI stability

Author: abose

src-mdviewer/src/bridge.js

@@ -808,10 +808,14 @@ function handleReloadFile(data) {
 
 function handleSetTheme(data) {
     const { theme } = data;
+    const newScheme = theme === "dark" ? "dark" : "light";
     // Skip if already applied to avoid reflows that can reset scroll position
-    if (document.documentElement.getAttribute("data-theme") === theme) return;
+    if (document.documentElement.getAttribute("data-theme") === theme &&
+        document.documentElement.style.colorScheme === newScheme) {
+        return;
+    }
     document.documentElement.setAttribute("data-theme", theme);
-    document.documentElement.style.colorScheme = theme === "dark" ? "dark" : "light";
+    document.documentElement.style.colorScheme = newScheme;
     setState({ theme });
 }
 

src-mdviewer/src/core/doc-cache.js

@@ -176,6 +176,10 @@ export function saveActiveScrollPos() {
     const entry = cache.get(activeFilePath);
     if (!entry) return;
 
+    // Don't overwrite scroll position if viewer is hidden (e.g. panel closed)
+    // — hidden elements report scrollTop = 0 which would destroy the saved value.
+    if (!viewerContainer.offsetParent && viewerContainer.scrollTop === 0) return;
+
     entry.scrollPos = viewerContainer.scrollTop;
 
     // Also save source line for reload scenarios (DOM rebuilt, pixel pos unreliable)

test/spec/md-editor-integ-test.js

@@ -774,12 +774,13 @@ define(function (require, exports, module) {
                 // Verify edit mode preserved
                 await _assertMdEditMode(true);
 
-                // Verify scroll position preserved (wider tolerance for CI)
+                // Verify scroll position preserved (wider tolerance for CI).
+                // Wait longer as theme/content sync after reopen can cause transient reflows.
                 await awaitsFor(() => {
                     const scroll = _getViewerScrollTop();
                     return scroll > 10 && Math.abs(scroll - scrollBefore) < 150;
-                }, "scroll position to be preserved after panel reopen", 5000);
-            }, 15000);
+                }, "scroll position to be preserved after panel reopen", 8000);
+            }, 20000);
 
             it("should reload button re-render current file with fresh DOM preserving scroll and edit mode", async function () {
                 await _openMdFileAndWaitForPreview("long.md");