Now, I tried this on 5.20 64x, no idea what is the behaviour in other versions.
Fact is, that in the Dosbox keymapper ( that you enter with Ctrl-F1) your mouse cursor stays invisible and you are not able to redefine keys. Now in general I directly edit the xxx.map file. But this is clear an error and the normal function not working in this dosbox version.
If you hold pressed the mouse button 1, you can see how the invisible hoover over the fields, turning them red.
Most of my mapper files dates from Recalbox 180713, I never had rthis problem before. The mouse is directly connected over USB, I wanted to be sure that it was not an transmission error.
Now, another problem is that some keystrikes or combinations INSIDE Dosbox still not valid for Dosbox.
As example if you push the key combination Alt+Ctrl+X in Larry 1 to skip age verification questions, Dosbox crash out to the Unix terminal. In my opinion key combinations from ES should be disabled inside running Dosbox game.
