QMainWindow: respect the maximum size of the central widget

Make sure the dock widgets get the available space when the
central widget cannot expand any more.

[ChangeLog][QtWidgets][QMainWindow] Dock widgets will now be resized
properly when the central widget has a fixed size.

Task-number: QTBUG-40410
Change-Id: Id06c07b79aa3102aa41212fa2c621f5fa426fe02
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>

src/widgets/widgets/qdockarealayout.cpp

@@ -2575,12 +2575,14 @@ void QDockAreaLayout::getGrid(QVector<QLayoutStruct> *_ver_struct_list,
 {
     QSize center_hint(0, 0);
     QSize center_min(0, 0);
+    QSize center_max(0, 0);
     const bool have_central = centralWidgetItem != 0 && !centralWidgetItem->isEmpty();
     if (have_central) {
         center_hint = centralWidgetRect.size();
         if (!center_hint.isValid())
             center_hint = centralWidgetItem->sizeHint();
         center_min = centralWidgetItem->minimumSize();
+        center_max = centralWidgetItem->maximumSize();
     }
 
     QRect center_rect = rect;
@@ -2656,7 +2658,7 @@ void QDockAreaLayout::getGrid(QVector<QLayoutStruct> *_ver_struct_list,
         left = (tl_significant && bl_significant) ? left_min.height() : 0;
         right = (tr_significant && br_significant) ? right_min.height() : 0;
         ver_struct_list[1].minimumSize = qMax(left, center_min.height(), right);
-        ver_struct_list[1].maximumSize = have_central ? QWIDGETSIZE_MAX : 0;
+        ver_struct_list[1].maximumSize = center_max.height();
         ver_struct_list[1].expansive = have_central;
         ver_struct_list[1].empty = docks[QInternal::LeftDock].isEmpty()
                                         && !have_central
@@ -2717,7 +2719,7 @@ void QDockAreaLayout::getGrid(QVector<QLayoutStruct> *_ver_struct_list,
         bottom = (bl_significant && br_significant) ? bottom_min.width() : 0;
         hor_struct_list[1].minimumSize = qMax(top, center_min.width(), bottom);
 
-        hor_struct_list[1].maximumSize = have_central ? QWIDGETSIZE_MAX : 0;
+        hor_struct_list[1].maximumSize = center_max.width();
         hor_struct_list[1].expansive = have_central;
         hor_struct_list[1].empty = !have_central;
         hor_struct_list[1].pos = center_rect.left();

tests/auto/widgets/widgets/qmainwindow/tst_qmainwindow.cpp

@@ -137,6 +137,7 @@ private slots:
 #endif
     void addToolbarAfterShow();
     void centralWidgetSize();
+    void fixedSizeCentralWidget();
     void dockWidgetSize();
     void QTBUG2774_stylechange();
     void QTBUG15080_restoreState();
@@ -1737,6 +1738,50 @@ void tst_QMainWindow::centralWidgetSize()
     QTRY_COMPARE(widget.size(), widget.sizeHint());
 }
 
+void tst_QMainWindow::fixedSizeCentralWidget()
+{
+    // QTBUG-40410: dock widgets does not get all the available space when
+    // central widget is fixed size
+    QMainWindow mainWindow;
+    mainWindow.setCorner(Qt::TopLeftCorner, Qt::LeftDockWidgetArea);
+
+    MyWidget widget;
+    widget.setFixedSize(100,100);
+    mainWindow.setCentralWidget(&widget);
+
+    QDockWidget dock("D1");
+    QWidget *child = new MyWidget;
+    dock.setWidget(child);
+    mainWindow.addDockWidget(Qt::TopDockWidgetArea, &dock);
+
+    QDockWidget dock2("D2");
+    dock2.setWidget(new MyWidget);
+    mainWindow.addDockWidget(Qt::LeftDockWidgetArea, &dock2);
+
+    QSize sizeH = mainWindow.sizeHint();
+    QSize mwSize = QSize(sizeH.width(), sizeH.height() * 2);
+    mainWindow.resize(mwSize);
+    mainWindow.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&mainWindow));
+    if (mainWindow.height() < mwSize.height())
+        QSKIP("The screen is too small for this test");
+
+    // first, check that we get more than the size hint when we have more space
+    QTRY_VERIFY(child->height() > child->sizeHint().height());
+    int childHeight = child->height();
+
+    if (qGuiApp->styleHints()->showIsFullScreen())
+        QSKIP("The platform is auto maximizing, so we cannot resize the window");
+
+    // then, check that we get nothing when there is no space
+    mainWindow.resize(100,100);
+    QTRY_COMPARE(child->height(), 0);
+
+    // finally verify that we get the space back when we resize to the old size
+    mainWindow.resize(mwSize);
+    QTRY_COMPARE(child->height(), childHeight);
+}
+
 void tst_QMainWindow::dockWidgetSize()
 {
     QMainWindow mainWindow;