Maximize QMdiSubWindow in QSplitter

Question

I have QSplitter set as the central widget:

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent),
      mdiArea(new QMdiArea)
{
    QWidget *widget = new QWidget;
    widget->setMinimumSize(100, 100);

    QSplitter *splitter = new QSplitter;
    splitter->addWidget(mdiArea);
    splitter->addWidget(widget);

    setCentralWidget(splitter);

    createActions();
}

void MainWindow::createSubwin()
{
    QWidget *subwin = new QWidget(mdiArea);
    subwin->setWindowTitle("Subwindow");
    subwin->setMinimumSize(100, 100);
    mdiArea->addSubWindow(subwin);

    subwin->show();
}

void MainWindow::createActions()
{
    QAction *actSub = new QAction("Add subwindow", this);
    connect(actSub, SIGNAL(triggered()), SLOT(createSubwin()));

    QMenu *winMenu = menuBar()->addMenu("Windows");
    winMenu->addAction(actSub);
}

When I press maximize button of subwindow, the subwindow covers entire main window. Is there any way to prevent such behaviour and make subwindow occupy all the space of QMdiArea instead?

UPD: It looks like that the problem occurs only when at least one menu in main window's QMenuBar is present. Without menuBar everything works as expected:

https://www.qtcentre.org/threads/44457-QMdiSubWindow-maximizing-problem

A simple idea: Move the right part of the `QSplitter` into a dock widget (and drop the `QSplitter`), so that the left part is the only part of the `QMainWindow::centralWidget()`. This would mean to work _with_ the existing class instead of _against_, and is probably easier to manage. — Scheff's Cat, Mar 12 '22 at 12:12
@Scheff'sCat Thank you for mentioning QDockWidget, it seems that it is much better than QSplitter in this case. Sadly, it still doesn't prevent QMdiSubWIndow from taking the entire space of the main window. — razor3x, Mar 12 '22 at 12:57
I do what you do but it works for me! I use `Qt 6.2.3` instead of `Qt 5.12.0`. means that without using `QDockWidget` — Parisa.H.R, Mar 12 '22 at 13:31

Scheff's Cat · Accepted Answer · 2022-03-12T15:31:33.587

Regarding the QSplitter, I gave OP the following hint:

Move the right part of the QSplitter into a dock widget (and drop the QSplitter), so that the left part is the only part of the QMainWindow::centralWidget(). This would mean to work with the existing class instead of against, and is probably easier to manage.

OP appreciated the hint with the dock widget but claimed the sub-window will still occupy the whole main window.

I must admit my lack of experience with MDI and made an MCVE to prove me myself right or wrong:

#include <QtWidgets>

// main application

int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QApplication app(argc, argv);
  // setup GUI
  QMainWindow qWinMain;
  qWinMain.setWindowTitle("QMainWindow - MDI - Dock");
  qWinMain.resize(640, 480);
  // MDI
  QMdiArea qMDI;
  qWinMain.setCentralWidget(&qMDI);
  // MDI sub widget
  QLabel qWinSub("MDI Sub-Window\nwidget");
  qMDI.addSubWindow(&qWinSub);
  // Dock
  QDockWidget qDock;
  qDock.setWindowTitle("Dock");
  QLabel qLblDock("Dock\nwidget");
  qDock.setWidget(&qLblDock);
  qWinMain.addDockWidget(Qt::RightDockWidgetArea, &qDock);
  qWinMain.show();
  // runtime loop
  return app.exec();
}

Output:

So, I cannot reproduce OPs claim—it works on my side.

My platform: Windows 10, VS2019, Qt5.15

I enhanced the first MCVE a bit to see how it works if MDI sub-windows are created after qWinMain.show() (what's expected as the usual case).

#include <QtWidgets>

// main application

int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QApplication app(argc, argv);
  // setup GUI
  QMainWindow qWinMain;
  qWinMain.setWindowTitle("QMainWindow - MDI - Dock");
  qWinMain.resize(640, 480);
  // MDI
  QMdiArea qMDI;
  qWinMain.setCentralWidget(&qMDI);
  // Dock
  QDockWidget qWinDock;
  qWinDock.setWindowTitle("Dock");
  QWidget qDock;
  QVBoxLayout qVBoxDock;
  QPushButton qBtnNewMDISubWin("New Sub-Window");
  qVBoxDock.addWidget(&qBtnNewMDISubWin);
  qDock.setLayout(&qVBoxDock);
  qWinDock.setWidget(&qDock);
  qWinMain.addDockWidget(Qt::RightDockWidgetArea, &qWinDock);
  // create sub-window
  int i = 0;
  auto createSubWin = [&]() {
    ++i;
    QLabel* pQWinSub = new QLabel(QString("MDI Sub-Window\nwidget %1").arg(i));
    pQWinSub->setWindowTitle(QString("MDI Sub-Window %1").arg(i));
    qMDI.addSubWindow(pQWinSub);
    pQWinSub->show();
  };
  // install signal handlers
  QObject::connect(&qBtnNewMDISubWin, &QPushButton::clicked,
    createSubWin);
  // runtime loop
  qWinMain.show();
  return app.exec();
}

Output:

It still works on my side as expected.

Note:

I had to add the explicit pQWinSub->show(); after qMDI.addSubWindow(pQWinSub); (which was not necessary in the first MCVE). However, this is exactly how it's done by OP's code.

OPs reply:

It turns out that the problem occurs only when menuBar is present

Oha. How comes?

I extended my MCVE again to add a menu bar:

#include <QtWidgets>

// main application

int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QApplication app(argc, argv);
  // setup GUI
  QMainWindow qWinMain;
  qWinMain.setWindowTitle("QMainWindow - MDI - Dock");
  qWinMain.resize(640, 480);
  // menu
  QMenuBar qMenuMain;
  QAction qCmdFile("File");
  QMenu qMenuFile;
  QAction qCmdFileNew("New");
  qMenuFile.addAction(&qCmdFileNew);
  qCmdFile.setMenu(&qMenuFile);
  qMenuMain.addAction(&qCmdFile);
  qWinMain.setMenuBar(&qMenuMain);
  // MDI
  QMdiArea qMDI;
  qWinMain.setCentralWidget(&qMDI);
  // Dock
  QDockWidget qWinDock;
  qWinDock.setWindowTitle("Dock");
  QWidget qDock;
  QVBoxLayout qVBoxDock;
  QPushButton qBtnNewMDISubWin("New Sub-Window");
  qVBoxDock.addWidget(&qBtnNewMDISubWin);
  qDock.setLayout(&qVBoxDock);
  qWinDock.setWidget(&qDock);
  qWinMain.addDockWidget(Qt::RightDockWidgetArea, &qWinDock);
  // create sub-window
  int i = 0;
  auto createSubWin = [&]() {
    ++i;
    QLabel* pQWinSub = new QLabel(QString("MDI Sub-Window\nwidget %1").arg(i));
    pQWinSub->setWindowTitle(QString("MDI Sub-Window %1").arg(i));
    pQWinSub->setFrameShape(QFrame::Box);
    qMDI.addSubWindow(pQWinSub);
    pQWinSub->show();
  };
  // install signal handlers
  QObject::connect(&qCmdFileNew, &QAction::triggered,
    createSubWin);
  QObject::connect(&qBtnNewMDISubWin, &QPushButton::clicked,
    createSubWin);
  // runtime loop
  qWinMain.show();
  return app.exec();
}

Output:

Note:

I partly agree with OP:

Yes, the look of the maximized MDI is a bit different now. It looks like it occupies the whole client area of the main window but…

…the dock widget is still visible. I added a box to the QLabel (the top widget in the MDI sub-window) to illustrate this. In fact, the sub-window still occupies the central widget only (regardless what the look of its title bar suggests).

Thank you so much, it also works in Qt 5.12.12. It turns out that the problem occurs only when menuBar is present — razor3x, Mar 12 '22 at 13:59

Maximize QMdiSubWindow in QSplitter

1 Answers1