Try the following sample. Type some letter in the "File Name" field, so that the completer pops up. Now, move the file dialog by dragging its caption bar: the dialog moves but the completer popup sticks.
This happens only on OSX. On Windows, the popup disappears as soon as clicking on the title bar.
My sample uses the file dialog to make an easy example, but this is not a file dialog issue, this is a QCompleter problem on MacOS.