since DT3 I need to rearrange the frame sizes more often and so the following problem is kind of annoying.
I am using the widescreen mode and find it hard to use the window separator as it’s to small or because the mouse courser is not changing to double arrow reliable.
It only changes when you move from the left frame (document list) to the right (document) but not vice versa. If you miss the pixel where the mouse arrow changes then you need to reposition the mouse on the leftern side.
Which version of macOS do you use? A screenshot of the window setup might be useful too as a quick test on Catalina worked in both directions but there are several separators and it’s unclear which one causes the troubles.