Properly register and dispose of majority of remaining listeners

src/utils/TestUtils.test.ts

@@ -346,6 +346,9 @@ export class MockRenderer implements IRenderer {
 }
 
 export class MockViewport implements IViewport {
+  dispose(): void {
+    throw new Error('Method not implemented.');
+  }
   scrollBarWidth: number = 0;
   onThemeChanged(colors: IColorSet): void {
     throw new Error('Method not implemented.');