Fix scrollback going missing when increase size on Windows

This also forces the scroll bar to sync after a resize early, this may fix
other bugs.

Fixes #2459

src/browser/Types.d.ts

@@ -35,7 +35,7 @@ export interface IPartialColorSet {
 
 export interface IViewport extends IDisposable {
   scrollBarWidth: number;
-  syncScrollArea(): void;
+  syncScrollArea(immediate?: boolean): void;
   getLinesScrolled(ev: WheelEvent): number;
   onWheel(ev: WheelEvent): boolean;
   onTouchStart(ev: TouchEvent): void;