[Qt] An event filter to keep `QMenu` open after an action it contains has been triggered.

StayOpenMenuFilter.h

#pragma once

#include <QApplication>
#include <QKeyEvent>
#include <QMenu>
#include <QStyle>

/**
	\class StayOpenMenuFilter
	\brief An event filter to keep a `QMenu` open after an action it contains has been triggered.
	
	When applied to a popup QMenu, will prevent closing the menu after an action is selected.
	Meant to be used with `QObject::installEventFilter(new StayOpenMenuFilter())`, but also
	has `installOnMenu()` convenience method, and will install itself if the `QObject *parent`
	passed to c'tor is an instance of `QMenu`.  E.g.:
	```
		QMenu *theMenu = new QMenu(this);
		new StayOpenMenuFilter(theMenu);
	```
	By default it will prevent closing the menu when any action that doesn't have a sub-menu
	is triggered. To only prevent closing when checkable actions are triggered, the public
	member `onlyCheckableActions` can be set to `true` (as 2nd argument to the c'tor or separately).
	
	Based on https://forum.qt.io/topic/13615/solved-how-to-keep-a-context-menu-open/5?_=1737767077720
	and https://stackoverflow.com/a/75275070/3246449
*/
class StayOpenMenuFilter : public QObject
{
	Q_OBJECT
	public:
		explicit StayOpenMenuFilter(QObject *parent = nullptr, bool onlyCheckableActions = false) :
		  QObject(parent),
		  onlyCheckableActions{onlyCheckableActions}
		{
			if (QMenu *m = qobject_cast<QMenu*>(parent))
				installOnMenu(m);
		}

		bool onlyCheckableActions {false};

		void installOnMenu(QMenu *w) {
			w->installEventFilter(this);
			connect(w, &QMenu::aboutToHide, this, [this]() { aboutToHide = true; });
			connect(w, &QMenu::aboutToShow, this, [this]() { aboutToHide = false; });
		}

	protected:
		bool eventFilter(QObject *obj, QEvent *ev) override
		{
			QMenu *menu;
			QAction *activeAction;

			if (
				aboutToHide ||
				!(menu = qobject_cast<QMenu*>(obj)) ||
				!(activeAction = menu->activeAction()) ||
				!activeAction->isEnabled() ||
				activeAction->isSeparator() ||
				!!activeAction->menu() ||
				(onlyCheckableActions && !activeAction->isCheckable())
			) {
				return QObject::eventFilter(obj, ev);
			}

			bool filter = false;
			if (ev->type() == QEvent::KeyPress) {
				const int key = static_cast<QKeyEvent*>(ev)->key();
				filter = (
					key == Qt::Key_Return ||
					key == Qt::Key_Enter ||
					(key == Qt::Key_Space && QApplication::style()->styleHint(QStyle::SH_Menu_SpaceActivatesItem, nullptr, menu))
				);
			}
			else if (ev->type() == QEvent::MouseButtonRelease) {
				filter = true;
				/* Technically on Windows only context menus can be activated with the right mouse button.
				   But determining what caused the menu is out of scope for now. See QMenu::mouseReleaseEvent() for example.
				#ifdef Q_OS_WIN
					filter = static_cast<QMouseEvent*>(ev)->button() == Qt::LeftButton || <is top level widget>;
				#else
					filter = true
				#endif */
			}

			if (filter) {
				activeAction->trigger();
				ev->accept();
				return true;
			}
			return QObject::eventFilter(obj, ev);
		}

		bool aboutToHide {false};
};