@JackMyson
You don't actually show the message box being shown (exec()?). Nothing special here. Try not subclassing the QMessageBox and using that directly, so you can rule that out. Try a standalone application without anything of your own. Try passing nullptr as parent to the constructor, any difference? Are you sure you are not using Wayland rather than Xorg, that might make its position or behaviour different.

@SGaist

My data is ready to append. So i append them directly. There are 10-100 QTextEdits. I append 100-200 line to each of QTextEdits. Lines have arround 300 characters. I have one vector of vectors source.

By the way, i moved my data source to a different thread. Then emitted signals to append with BlockingQueuedConnection. Gui is responsive now. But its not smooth. I dont know using blockingqueuedconnection instead of queuedconnection is the right way but my gui is responsive while appending strings now.

@aha_1980 Here's how I bootstrapped new working settings for QtCreator.
On Windows the settings reside in <user>\Appdata\Roaming\QtProject, which contains files QtCreator.db, QtCreator.ini and directory qtcreator with lots of subdirectories, xml files etc.
Renaming that directory then running QtCreator created a new settings directory (from a template kept in the Qt distro) . After that, QtCreator started behaving normally, though of course without any memory of previous projects or kit associations.
Copying the QtCreator.ini file from the renamed directory restored those memories but did not make the bad behavior come back.
Apparently the root problem is an incompatibility between Qt Creator 4.10.2 and some old file in the settings folder. In my case those files were created years ago by QtCreator 3.5. And since 3.5 does not even support VC 2015, I guess it was high time for a change.