QT creator Ubuntu why doesn't it close with a cross?

Question

Qt Creator

when you click the cross in the upper right corner, he first closed the document, but he remained
and now there is simply no reaction.
except for the program bar - close the window - I did not find how to close it.
Why doesn't it close with a cross?

Answer 1

Now I understand - just not the whole window was on the screen. the closing cross is above the cross.
but this tryndets had to be thought of