Fix markdown webview state updates (microsoft#178153)

Fixes microsoft#164071

Also fixes the webview not scrolling when the resources changes

Author: mjbvz

extensions/markdown-language-features/preview-src/index.ts

@@ -23,14 +23,13 @@ let documentResource = settings.settings.source;
 const vscode = acquireVsCodeApi();
 
 const originalState = vscode.getState() ?? {} as any;
-
 const state = {
-	originalState,
+	...originalState,
 	...getData<any>('data-state')
 };
 
-if (originalState?.resource !== state.resource) {
-	state.scrollProgress = undefined;
+if (typeof originalState.scrollProgress !== 'undefined' && originalState?.resource !== state.resource) {
+	state.scrollProgress = 0;
 }
 
 // Make sure to sync VS Code state here
@@ -67,7 +66,9 @@ onceDocumentLoaded(() => {
 	if (typeof scrollProgress === 'number' && !settings.settings.fragment) {
 		doAfterImagesLoaded(() => {
 			scrollDisabledCount += 1;
-			window.scrollTo(0, scrollProgress * document.body.clientHeight);
+			// Always set scroll of at least 1 to prevent VS Code's webview code from auto scrolling us
+			const scrollToY = Math.max(1, scrollProgress * document.body.clientHeight);
+			window.scrollTo(0, scrollToY);
 		});
 		return;
 	}