Fix floating window position when dragging with constraints

When the detach starts we might make the window smaller so it
respects the max-size constraint. The quirk was that in that
case the window was no longer under the mouse cursor, so looked
weird while dragging, although it worked

src/DockWidgetBase.cpp

@@ -273,7 +273,7 @@ void DockWidgetBase::setFloating(bool floats)
         auto lastGeo = lastPositions().lastFloatingGeometry();
         if (lastGeo.isValid()) {
             if (auto fw = floatingWindow())
-                fw->setSuggestedGeometry(lastGeo);
+                fw->setSuggestedGeometry(lastGeo, /*preserveCenter=*/true);
         }
     } else {
         saveLastFloatingGeometry();

src/private/DragController.cpp

@@ -184,6 +184,16 @@ void StateDragging::onEntry(QEvent *)
     q->m_windowBeingDragged = q->m_draggable->makeWindow();
     if (q->m_windowBeingDragged) {
         qCDebug(state) << "StateDragging entered. m_draggable=" << q->m_draggable << "; m_windowBeingDragged=" << q->m_windowBeingDragged->floatingWindow();
+
+        auto fw = q->m_windowBeingDragged->floatingWindow();
+        if (!fw->geometry().contains(q->m_pressPos)) {
+            // The window shrunk when the drag started, this can happen if it has max-size constraints
+            // we make the floating window smaller. Has the downside that it might not be under the mouse
+            // cursor anymore, so make the change
+            if (fw->width() < q->m_offset.x()) { // make sure it shrunk
+                q->m_offset.setX(fw->width() / 2);
+            }
+        }
     } else {
         // Shouldn't happen
         qWarning() << Q_FUNC_INFO << "No window being dragged for " << q->m_draggable->asWidget();

src/private/FloatingWindow.cpp

@@ -221,7 +221,7 @@ const Frame::List FloatingWindow::frames() const
     return m_dropArea->findChildren<Frame *>(QString(), Qt::FindDirectChildrenOnly);
 }
 
-void FloatingWindow::setSuggestedGeometry(QRect suggestedRect)
+void FloatingWindow::setSuggestedGeometry(QRect suggestedRect, bool preserveCenter)
 {
     const Frame::List frames = this->frames();
     if (frames.size() == 1) {
@@ -236,7 +236,8 @@ void FloatingWindow::setSuggestedGeometry(QRect suggestedRect)
         // Resize to new size but preserve center
         const QPoint originalCenter = suggestedRect.center();
         suggestedRect.setSize(size);
-        suggestedRect.moveCenter(originalCenter);
+        if (preserveCenter)
+            suggestedRect.moveCenter(originalCenter);
     }
 
     setGeometry(suggestedRect);

src/private/FloatingWindow_p.h

@@ -69,8 +69,11 @@ public:
      * @brief Equivalent to setGeometry(), but the value might be adjusted.
      *
      * For example, if the suggestedRect is bigger than max size, we'll make it smaller.
+     *
+     * @param preserveCenter, if true, then the center is preserved
+     *
      */
-    void setSuggestedGeometry(QRect suggestedRect);
+    void setSuggestedGeometry(QRect suggestedRect, bool preserveCenter = false);
 
     bool anyNonClosable() const;
     bool anyNonDockable() const;