I saw this bug has been closed by saying that is a graphic card problem, but in my case is not.
I have used the TMXimporter plugin and after pasting a VERY long text, the textbox has been resized and all the buttons have gone down outside the screen.
After that I lost every back/done button, like if they have been left at the same position outside the screen.
It will happen in any case where you paste/write extremely long text in a parameter textbox (or any other textbox in the events sheet).
Anyways, wouldn't be better if the textbox shows the scrollbars if became too big instead of going outside the screen? :D
I hope this could help :)