I guess that is because the mouse settings might be too sensible. Anyhow I uploaded a new version which changes the field only if the mouse cursor entered a new field or if the mouse button has been pressed. Probably this mitigates the issue.
That the mouse follows the cursor keys is mame specific. There is a setting that the mouse follows the cursor keys (INC/DEC). You can disable it: press TAB, "Input Settings", "Input Assignments (this system)". Scroll down to "Mouse X Analog Inc". There is the key (Right) defined which moves the mouse by one. Press delete to undefine the key. Do the same for X Dec and Y Inc/Dec. Close the settings with TAB.
You might experience that the TAB key is not working, then on Windows you need to press Scroll-Lock before (on Mac FN+Delete/Backspace).