src/vs/workbench/browser/parts/editor/editorGroupView.ts/EditorGroupView/shouldConfirmClose
class EditorGroupView:	private shouldConfirmClose(editor: EditorInput): boolean {
		if (editor.closeHandler) {
			return editor.closeHandler.showConfirm(); // custom handling of confirmation on close
		}

		return editor.isDirty() && !editor.isSaving(); // editor must be dirty and not saving
	}