doc: Explain the use of QContextMenuEvent cross-platform

...and across Qt Quick and Widgets, and somewhat from a usability
perspective.

Task-number: QTBUG-67331
Task-number: QTBUG-93486
Change-Id: I75085f3a7a8e76c3e67ff5864600324faf262a76
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
(cherry picked from commit 6e089a66a4d08a5d788c194560fccdec793e6337)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qevent.cpp

@@ -1907,14 +1907,65 @@ Q_IMPL_EVENT_COMMON(QIconDragEvent)
 
     \ingroup events
 
-    Context menu events are sent to widgets when a user performs
-    an action associated with opening a context menu.
-    The actions required to open context menus vary between platforms;
-    for example, on Windows, pressing the menu button or clicking the
-    right mouse button will cause this event to be sent.
+    A context menu event is sent when a user performs an action that should
+    open a contextual menu:
+    \list
+    \li clicking the right mouse button
+    \li pressing a dedicated keyboard menu key (if the keyboard has one,
+        such as the menu key on standard 104-key PC keyboards)
+    \li pressing some other keyboard shortcut (such as "Ctrl+Return" by
+        default on macOS 15 and newer)
+    \endlist
 
-    When this event occurs it is customary to show a QMenu with a
-    context menu, if this is relevant to the context.
+    The expected context menu should contain \l {QAction}{actions} that are
+    relevant to some content within the application (the "context"). In Qt, the
+    context is at least the particular \l {QWidget}{widget} or Qt Quick \l Item
+    that receives the QContextMenuEvent. If there is a selection, that should
+    probably be treated as the context. The context can be further refined
+    using \l QContextMenuEvent::pos() to pinpoint the content within the
+    widget, item or selection.
+
+    Widgets can override \l QWidget::contextMenuEvent() to handle this event.
+    Many widgets already do that, and have useful context menus by default.
+    Some widgets have a function such as
+    \l {QLineEdit::createStandardContextMenu()}{createStandardContextMenu()}
+    to populate the default set of actions into a \l QMenu, which can be
+    customized further in your subclass and then shown.
+
+    In Qt Quick, the event can be handled via the
+    \l {QtQuick.Controls::}{ContextMenu} attached property. Some
+    \l {QtQuick.Controls} Controls already provide context menus by default.
+
+    Unlike most synthetic events (such as a QMouseEvent that is sent only after
+    a QTouchEvent or QTabletEvent was not accepted), QContextMenuEvent is sent
+    regardless of whether the original mouse or key event was already handled
+    and \l {QEvent::isAccepted()}{accepted}. This is to accommodate the Windows
+    UI pattern of selecting some kind of items (icons, drawing elements, or
+    cells in an Item View) using the right mouse button (clicking or dragging),
+    and then getting a context menu as soon as you release the right mouse
+    button. (The actions on the menu are meant to apply to the selection.)
+    Therefore, on Windows the QContextMenuEvent is sent on mouse release; while
+    on other platforms, it's sent on press. Qt follows the
+    \l {QStyleHints::contextMenuTrigger()}{platform convention} by default.
+
+    There are also some Qt Quick Controls such as \l {QtQuick.Controls::}{Pane}
+    that accept mouse events, and nevertheless receive a QContextMenuEvent
+    after a mouse press or click.
+
+    If you prefer to support the press-drag-release UI pattern to open a
+    context menu on press, and drag over a menu item to select it on release,
+    you will need to do that by handling \l {QMouseEvent}{QMouseEvents} directly
+    (by overriding \l {QWidget::mousePressEvent()}{virtual functions} in
+    QWidget subclasses, or using \l TapHandler to open a \l Menu in Qt Quick);
+    and then the QContextMenuEvent will be redundant when the \l reason() is
+    \c Mouse. You should \l ignore() the event in that case; but you should
+    still ensure that the widget, custom control or application can respond to
+    a QContextMenuEvent that \l {reason()}{comes from} the platform-specific
+    keyboard shortcut.
+
+    When a QContextMenuEvent is \l {ignore()}{ignored}, Qt attempts to deliver
+    it to other widgets and/or Items under the \l {pos()}{position} (which
+    is usually translated from the cursor position).
 */
 
 #ifndef QT_NO_CONTEXTMENU
@@ -1968,6 +2019,9 @@ QContextMenuEvent::QContextMenuEvent(Reason reason, const QPoint &pos)
     Returns the position of the mouse pointer relative to the widget
     that received the event.
 
+    \note If the QContextMenuEvent did not come from the right mouse button,
+    \c pos() may be \l {QPoint::isNull()}{null}.
+
     \sa x(), y(), globalPos()
 */