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Window Geometry

Overview

QWidget provides several functions that deal with a widget's geometry. Some of these functions operate on the pure client area (i.e. the window excluding the window frame), others include the window frame. The differentiation is done in a way that covers the most common usage transparently.

Note that the distinction only matters for decorated top-level widgets. For all child widgets, the frame geometry is equal to the widget's client geometry.

This diagram shows most of the functions in use:

Unix/X11 peculiarities

On Unix/X11, a window does not have a frame until the window manager decorates it. This happens asynchronously at some point in time after calling show() and the first paint event the window receives: or it does not happen at all. Bear in mind that X11 is policy-free (others call it flexible). Thus you cannot make any safe assumption about the decoration frame your window will get. Basic rule: there's always one user who uses a window manager that breaks your assumption, and who will complain to you.

Furthermore, a toolkit cannot simply place windows on the screen. All Qt can do is to send certain hints to the window manager. The window manager, a separate process, may either obey, ignore or misunderstand them. Due to the partially unclear Inter-Client Communication Conventions Manual (ICCCM), window placement is handled quite differently in existing window managers.

X11 provides no standard or easy way to get the frame geometry once the window is decorated. Qt solves this problem with nifty heuristics and clever code that works on a wide range of window managers that exist today. Don't be surprised if you find one where frameGeometry() returns bogus results though.

Nor does X11 provide a way to maximize a window. The showMaximized() function in Qt therefore has to emulate the feature. Its result depends on the result of frameGeometry() and the capability of the window manager to do proper window placement, neither of which can be guaranteed.

Restoring a Window's Geometry

A common task in modern applications is to restore a window's geometry in a later session. On Windows, this is basically storing the result of geometry() and calling setGeometry() in the next session before calling show(). On X11, this won't work because an invisible window doesn't have a frame yet. The window manager would decorate the window later. When this happens, the window shifts towards the bottom/right corner of the screen depending on the size of the decoration frame. X theoretically provides a way to avoid this shift. Our tests have shown, though, that almost all window managers fail to implement this feature.

A workaround is to call setGeometry() after show(). This has the two disadvantages that the widget appears at a wrong place for a millisecond (results in flashing) and that currently only every second window manager gets it right. A safer solution is to store both pos() and size() and to restore the geometry using resize() and move() before calling show(), as demonstrated in the following example:

    MyWidget* widget = new MyWidget
    ...
    QPoint p = widget->pos();   // store position
    QSize s = widget->size();   // store size
    ...
    widget = new MyWidget;
    widget->resize( s );        // restore size
    widget->move( p );          // restore position
    widget->show();             // show widget

This method works on both MS-Windows and most existing X11 window managers.