Ok, I have now created a PoC exe file instead, and emailed it (together with the other requested information) according to your request above.
Also, one more important thing I noticed about the bug is the following:
When you execute the PoC external tool using a shortcut, the Output Bar will appear and the GUI focus in EmEditor will go to that (I don’t mean that this is part of the problem, it’s just the normal behavior leading up to my next step in the description here below).
If you then click the editor part of the GUI once after this, the cursor will appear and you will seemingly be back in the editor, and this is when the cursor will behave strangely while the PoC program is running (try to move the cursor with the arrow keys at that point, for example to the left, and you will see).
BUT, if you click the editor window once again, the cursor problem will disappear!
That is, important to note:
This bug only happens if you click the editor part of the GUI once to get back to it from the Output Bar while the external tool is running, BUT if you click it again, the bug will disappear and you won’t be able to reproduce it!