extensions/typescript-language-features/src/utils/activeJsTsEditorTracker.ts/ActiveJsTsEditorTracker/onDidChangeActiveTextEditor
class ActiveJsTsEditorTracker:	private onDidChangeActiveTextEditor(editor: vscode.TextEditor | undefined): any {
		if (editor === this._activeJsTsEditor) {
			return;
		}

		if (editor && !editor.viewColumn) {
			// viewColumn is undefined for the debug/output panel, but we still want
			// to show the version info for the previous editor
			return;
		}

		if (editor && this.isManagedFile(editor)) {
			this._activeJsTsEditor = editor;
		} else {
			this._activeJsTsEditor = undefined;
		}
		this._onDidChangeActiveJsTsEditor.fire(this._activeJsTsEditor);
	}