Created attachment 111912 [details] Minimal bug reproduction project The desktop freezes when I try to copy to clipboard and send a notification in my program. It doesn't freeze completely, you can still move windows and use some programs but the freeze is severe and can last between 10 and 30 secs. I've created a project in QT in order to ease the testing process. It creates a QWidget with a QPushButton, when you press it the desktop freezes. See the attachment.
I can reproduce with your example. The problem is that you set a huuuuge pixmap into the clipboard and then Qt freezes in QMimeData (more specifically QXcbClipboardMime) trying to read the clipboard contents. The fact that a notification freezes the desktop is that when a notification is shown, the text field checks clipboard to see whether the "Paste" action (to make notification text selectable it is a text field and not a plain label) should be enabled. I would consider this an X or Qt bug.
Yeah, please report to the Qt people. Thanks!