When I tried to select code in the CSS editor, it would sometimes close.
I was not able to reproduce this error during the recording so I was not able to find an exact reason for it.
This happened several times and I think mostly when I had an incomplete line (error) and wanted to select that line. Plus I left my curser within the limits of the editor!
I’m not sure where I reproduced this error, but I can confirm that it occurs in the Cwicly Template Parts area. I think it happens most of the time when the left mouse button is released in the red area of the image below when selecting code.
I can confirm this behaviour, although specifically related to Firefox.
I’m not able to reproduce the issue when kept within the code editor, your screenshots show the same. Can you confirm that you don’t encounter the issue when keeping your selection within the code editor?