Fix markdown scrolling between elements being too jumpy (microsoft#166746)

Fixes microsoft#165055

We need to consider the distance between the end of the previous element and start of the next element. Previously we were taking distance from start of the previous to start of the next

Author: mjbvz

extensions/markdown-language-features/preview-src/scroll-sync.ts

@@ -133,8 +133,9 @@ export function scrollToRevealSourceLine(line: number, documentVersion: number,
 	if (next && next.line !== previous.line) {
 		// Between two elements. Go to percentage offset between them.
 		const betweenProgress = (line - previous.line) / (next.line - previous.line);
-		const elementOffset = next.element.getBoundingClientRect().top - previousTop;
-		scrollTo = previousTop + betweenProgress * elementOffset;
+		const previousEnd = previousTop + rect.height;
+		const betweenHeight = next.element.getBoundingClientRect().top - previousEnd;
+		scrollTo = previousEnd + betweenProgress * betweenHeight;
 	} else {
 		const progressInElement = line - Math.floor(line);
 		scrollTo = previousTop + (rect.height * progressInElement);