src/vs/workbench/services/editor/browser/editorService.ts/EditorService/doHandleActiveEditorChangeEvent
class EditorService:	private doHandleActiveEditorChangeEvent(): void {

		// Remember as last active
		const activeGroup = this.editorGroupService.activeGroup;
		this.lastActiveEditor = withNullAsUndefined(activeGroup.activeEditor);

		// Fire event to outside parties
		this._onDidActiveEditorChange.fire();
	}