1st defect found already !!!!
When using the latest build (11099) with a 4K external monitor as primary / only monitor - after a while the taskbar disappears and the start menu is also gone.
This defect doesn't seem to happen if 1) you set the 4K monitor as a SECONDARY monitor, or 2) you put the taskbar at either the left or right hand edge of the screen (i.e not in the usual places top / bottom).
However the file progress bar on copy / paste is now working again.