Ensure viewport is correctly reset on term.reset

Part of #4603

src/browser/Terminal.ts

@@ -1255,13 +1255,13 @@ export class Terminal extends CoreTerminal implements ITerminal {
     super.reset();
     this._selectionService?.reset();
     this._decorationService.reset();
+    this.viewport?.reset();
 
     // reattach
     this._customKeyEventHandler = customKeyEventHandler;
 
     // do a full screen refresh
     this.refresh(0, this.rows - 1);
-    this.viewport?.syncScrollArea();
   }
 
   public clearTextureAtlas(): void {

src/browser/TestUtils.test.ts

@@ -324,6 +324,9 @@ export class MockViewport implements IViewport {
   public scrollLines(disp: number): void {
     this._onRequestScrollLines.fire({ amount: disp, suppressScrollEvent: false });
   }
+  public reset(): void {
+    throw new Error('Method not implemented.');
+  }
 }
 
 export class MockCompositionHelper implements ICompositionHelper {

src/browser/Types.d.ts

@@ -161,13 +161,14 @@ export interface IPartialColorSet {
 export interface IViewport extends IDisposable {
   scrollBarWidth: number;
   readonly onRequestScrollLines: IEvent<{ amount: number, suppressScrollEvent: boolean }>;
-  syncScrollArea(immediate?: boolean): void;
+  syncScrollArea(immediate?: boolean, force?: boolean): void;
   getLinesScrolled(ev: WheelEvent): number;
   getBufferElements(startLine: number, endLine?: number): { bufferElements: HTMLElement[], cursorElement?: HTMLElement };
   handleWheel(ev: WheelEvent): boolean;
   handleTouchStart(ev: TouchEvent): void;
   handleTouchMove(ev: TouchEvent): boolean;
   scrollLines(disp: number): void;  // todo api name?
+  reset(): void;
 }
 
 export interface ILinkifierEvent {

src/browser/Viewport.ts

@@ -81,13 +81,25 @@ export class Viewport extends Disposable implements IViewport {
     this.register(this._optionsService.onSpecificOptionChange('scrollback', () => this.syncScrollArea()));
 
     // Perform this async to ensure the ICharSizeService is ready.
-    setTimeout(() => this.syncScrollArea(), 0);
+    setTimeout(() => this.syncScrollArea());
   }
 
   private _handleThemeChange(colors: ReadonlyColorSet): void {
     this._viewportElement.style.backgroundColor = colors.background.css;
   }
 
+  public reset(): void {
+    this._currentRowHeight = 0;
+    this._currentDeviceCellHeight = 0;
+    this._lastRecordedBufferLength = 0;
+    this._lastRecordedViewportHeight = 0;
+    this._lastRecordedBufferHeight = 0;
+    this._lastTouchY = 0;
+    this._lastScrollTop = 0;
+    // Sync on next animation frame to ensure the new terminal state is used
+    this._coreBrowserService.window.requestAnimationFrame(() => this.syncScrollArea());
+  }
+
   /**
    * Refreshes row height, setting line-height, viewport height and scroll area height if
    * necessary.