In the GUI texture viewer of the GUI editor, in which you set the texture coordinates, the SelectedHover tab was disabled. I am not sure why, but to re-enable it, open the guixml script _cleanenginetextureviewer.xml and on line 249:
<_tab name='selectedHover' enabled='false'>
remove the enabled='false' part, compile and submit.
I also adjust the size of the window to better fit my standard viewport, but that's just me.
If you make this change, then like changing any clean engine script, make note of it so it can be restored if an engine update overwrites it in the future.