inform QWidget that the screen dimensions changed

[Dzha]

The window does not check anything. It's your (or the window manager's) duty to know the screen size has changed.

OK. Is there a mechanism for informing the windows manager that the screen is changed?

[wysota]

You can start by reading what a window manager is.

[Dzha]

Well, all I was able to find while reading the window manager documentation was related to resizing the window, which I want to avoid.
I can not see any method of not leaving the full screen mode (thus avoiding additional repaint) that is described in Qt doc.

If you are aware of it, please tell me.

[wysota]

Well, all I was able to find while reading the window manager documentation

Which window manager documentation was that?

[Dzha]

QWS, Qt for Embedded Linux.

[wysota]

Are you using QWS?

[Dzha]

Yes, my main app acts as a QWS server. loads a screen plugin and uses it.

[wysota]

If your application is full screen, what do you need QWS for?

[Dzha]

QWS also handles dialogs, input, etc.
My main window is an instance of QMainWindow .

[wysota]

So how do you implement screen geometry change?

[Dzha]

So how do you implement screen geometry change?

Right, the orientation is implemented in the hardware. The app commands the driver to rotate, the driver changes how pixels are written to the screen and changes what geometry (w,h) is reported to the screen driver plugin. As a result, the dimensions of the screen are different, but the full screen window does not pick them up until it re-enters the full screen mode.
So only the refresh of the screen width and height is required.