extensions/typescript-language-features/src/utils/activeJsTsEditorTracker.ts/ActiveJsTsEditorTracker/constructor
class ActiveJsTsEditorTracker:	public constructor() {
		super();
		vscode.window.onDidChangeActiveTextEditor(this.onDidChangeActiveTextEditor, this, this._disposables);
		vscode.window.onDidChangeVisibleTextEditors(() => {
			// Make sure the active editor is still in the visible set.
			// This can happen if the output view is focused and the last active TS file is closed
			if (this._activeJsTsEditor) {
				if (!vscode.window.visibleTextEditors.some(visibleEditor => visibleEditor === this._activeJsTsEditor)) {
					this.onDidChangeActiveTextEditor(undefined);
				}
			}
		}, this, this._disposables);

		this.onDidChangeActiveTextEditor(vscode.window.activeTextEditor);
	}