This bug can be seen in the Qt MDI example (Qt...\examples\mainwindows\mdi).

It is reproducible on UNIX only.

Steps to reproduce:
1 - Compile and run the example.
2 - Create a new document (File->New).
3 - Enter some text into this new document.
4 - Resize the application to be as small as it possible can by grabbing a corner and dragging it towards its opposite corner.
5 - Notice that the "edit" toolbar now has an expand button (>>).
6 - Highlight text within the document.
7 - Click the expand button on the "edit" toolbar.
8 - Click and hold the Paste button. Do not release.
9 - While still clicked, drag the cursor out of the "edit" toolbar.
10 - Now release the click.
11 - Resize the application to be larger. Large enough so that the toolbars do not require an expand button.
12 - Notice that the Paste button remains depressed.