Unable to resize windows on split full screen in MacOS Mojave when using external monitor I'm running MacOS Mojave on a 13" Macbook pro with a 4K external monitor (scaled to 2560 x 1440).

When running two apps in split full screen mode it won't let me reposition the "split" which controls the share of the screen of the two apps by grabbing the vertical bar.

This behaviour can be seem in this screen recording.

Any ideas on how to solve this? Or is this just a bug that needs fixing?
WORKAROUND:
Read somewhere that to get (temporarily) the resizing bar, you drag one of the split windows by the title to the other side of the monitor. The bar will remain for a while. Resize, and if needed, switch back the windows. It has been working for me on an external LG monitor in Mojave.