I was often complaining about the front end performance under Linux, but it seems I could pin down a very critical issue. I made a screen-cast that shows how to reproduce the issue

- [Screen-cast on YouTube](https://youtu.be/nyNl7f9ZvAA)

How to reproduce: Open a fresh *Mathematica* and type some code. In the screen-cast I used

    Hold[Compile[{x, y}, x + y, CompilationTarget -> "C"]]

**Don't** copy it from here, type it! Evaluate the cell and then just use the arrow keys left and right to scroll though the code. It should go fast and as soon as you release the arrow key, the moving cursor should stop too. 

Now, **copy** the code from here and insert it into the notebook as very first cell. If you now move around, it will go noticeably slower and the there will be a large delay between the release of the arrow key and the stop of the cursor.

Now, use the left arrow key to go through the very beginning of the notebook. It's important that you *hold down* the arrow key for about 5-10 seconds even when you reached the beginning. 

At this point, the front end should be in a very weird state. Either, it inserts left arrow keys only, no matter which key you press, or it just eats some of your keys so thyou endp writngli ths...

Please, pretty please comment, when you can (even partially) reproduce this.