To reproduce:
1) Move over a deactivated button in any GNOME app.
2) Make the button active (by any other action)
Result: you can't click it. You have to move outside and inside again.

Example (just an example):
1) In gedit, place your mouse over the Undo button.
2) Write in the text field => The Undo button becomes active. 
3) Click.
Result: Nothing happens. You have to move the cursor outside and inside again.


Other information:
Hey, sorry for that bug report, because I don't know where to report it. 
This is an issue for all GNOME applications and I don't know if it is concerned
to GTK or X or anything. 
Please tell me, where is the right place to post this.