Strange delayed painting with Qt5 on Windows

Question

I'm using Qt5 in Windows. I just created a simple little widgets project in Qt Creator. I have a QMainWindow with a text edit widget inside. I enabled vertical layout, so the text edit consumes the full size of the inside of the main window (which is what I want, I'm trying to create a small notepad app).

As I drag the bottom right corner of the main window during the preview (I click the green triangle in the bottom left) I'm seeing a slight delay in the child widget's resizing. It doesn't exactly resize with the parent window on the same render frame (it seems like it is 1-2 render frames behind).

I remember years ago dealing with render lag like this in old school Win32 API. I remember I had to do double-buffered rendering into an offscreen bitmap to fix it (or something along those lines; been a long time).

Is there a way to fix this "render lag" in Qt? I can only imagine this is specific to Windows and might not impact other platforms, although I have not tested. If I need to provide more information let me know.

Answer 1

It is likely a Windows problem, not Qt's. The more GUI-heavy your window is the more noticeable it is.

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I investigated the same issue a while ago. We had a rather GUI-heavy window, with several widgets displaying 2D and 3D views of data. Resizing the window using lower-right corner resulted in a resize-redraw horror. Unfortunately it looks like the problem is not Qt related but rather the way that Windows handles redrawing a resized window. I was able to notice the problem even in the file explorer on Windows 7. Qt is indeed using double buffering by default (as mentioned in the comment by @Bim). I also tried explicitly triggering Qt's repaint events. This helped a little, but did not solve the problem. So after many efforts we just decided to live with it.