Fix preview pane scrolling to bottom on checkbox click

src/scroll.ts

@@ -8,20 +8,34 @@ export function startObserving(sourcePane: HTMLElement, targetPane: HTMLElement)
   }
 
   if ('onscrollend' in window) {
-    sourcePane.addEventListener('scrollend', () => syncScrollProgress(sourcePane, targetPane));
+    sourcePane.addEventListener('scrollend', () => {
+      if (!states.syncSuppressed) {
+        syncScrollProgress(sourcePane, targetPane);
+      }
+    });
   } else {
     sourcePane.addEventListener('scroll', () => {
       if (states.scrollUpdater !== undefined) {
         clearTimeout(states.scrollUpdater);
       }
 
       states.scrollUpdater = setTimeout(() => {
-        syncScrollProgress(sourcePane, targetPane);
+        if (!states.syncSuppressed) {
+          syncScrollProgress(sourcePane, targetPane);
+        }
       }, 100);
     });
   }
 }
 
+export function suppressScrollSync() {
+  states.syncSuppressed = true;
+}
+
+export function resumeScrollSync() {
+  states.syncSuppressed = false;
+}
+
 export function syncScrollProgress(sourcePane: HTMLElement, targetPane: HTMLElement, animated = true) {
   const { line, progress } = getScrollProgress(sourcePane);
   scrollToProgress(targetPane, line, progress, animated);
@@ -136,6 +150,8 @@ function clampProgressValue(value: number) {
 
 const states: {
   scrollUpdater: ReturnType<typeof setTimeout> | undefined;
+  syncSuppressed: boolean;
 } = {
   scrollUpdater: undefined,
+  syncSuppressed: false,
 };

src/view.ts

@@ -5,7 +5,7 @@ import { renderMarkdown, renderMermaid, renderKatex, handlePostRender, applyStyl
 import { replaceImageURLs } from './features/image';
 import { hidePreviewButtons, previewModes } from './support/settings';
 import { localized } from './shared/strings';
-import { syncScrollProgress } from './scroll';
+import { syncScrollProgress, suppressScrollSync, resumeScrollSync } from './scroll';
 import { resolveTaskToggle } from './features/task';
 import { ClassNames, CacheKeys } from './shared/const';
 
@@ -377,10 +377,15 @@ function handleTaskItemToggle(event: MouseEvent) {
 
   // Let the native toggle stand for instant feedback; just sync the source
   const from = lineRange.from + toggle.offset;
+
+  // Suppress scroll-sync for this dispatch: the editor's scrollDOM can emit a
+  // scrollend event that would jump the preview to the editor's cursor line.
+  suppressScrollSync();
   MarkEdit.editorView.dispatch({
     changes: { from, to: from + 1, insert: toggle.replacement },
     annotations: silentChange.of(true),
   });
+  requestAnimationFrame(resumeScrollSync);
 }
 
 const states: {