Desktop: Resolves laurent22#4827: Laggy scrolling in Markdown viewer (l…

…aurent22#5496)

packages/app-desktop/gui/note-viewer/index.html

@@ -357,6 +357,17 @@
 			return m ? contentElement.scrollTop / m : 0;
 		}
 
+		contentElement.addEventListener('wheel', webviewLib.logEnabledEventHandler(e => {
+			// When zoomFactor is not 1 (using an HD display is a typical case), 
+			// DOM element's scrollTop is incorrectly calculated after wheel scroll events 
+			// in the layer of Electron/Chromium, as of 2021-09-23.
+			// To avoid this problem, prevent the upstream from calculating scrollTop and
+			// calculate by yourself by accumulating wheel events.
+			// https://github.com/laurent22/joplin/pull/5496
+			contentElement.scrollTop = Math.max(0, Math.min(maxScrollTop(), contentElement.scrollTop + e.deltaY));
+			e.preventDefault();
+		}));
+
 		contentElement.addEventListener('scroll', webviewLib.logEnabledEventHandler(e => {
 			// If the last scroll event was done by the user, lastScrollEventTime is set and
 			// we can use that to skip the event handling. We skip it because in that case