Convention is that the arrow cursor will change to the cursor with a vertical line with left and right arrows.
There are times that I can't pick up the line even though my cursor is visually in the correct position. Unsure why. (Yes, tested this when the line was in the adjustable range. i.e. not at the max left position.)
If you can't delete this then perhaps this should be marked "existing feature" or "implemented." And transfer it to a bug report as per my earlier comment.
Can be deleted because now the correct cursor appears. Unsure why the behavior is spotty. I initially was annoyed because I couldn't move the divider. Later discovered that it worked reliably at the top of the divider, just right of the calendar (configure scans) icon. Tested before writing this and found that it was spotty with only the normal arrow cursor, but a few minutes ago, it was working perfectly. a glitch in the matrix. (Using the latest trial 6.11 if that makes any difference., on a Lenovo T430s i5 16GB win 10 1909)
Have had the same behavior with the horizontal divider on the Similar Folders tab. I moved it up but then could not move it back down. I had to go to another tab and then come back for it to start working again.