Here is one way to duplicate the problem...
Notebook with an external monitor (in my case the external monitor is 4K but shouldn't matter and the notebook monitor is HD).
Notebook set at 150% display scaling (in my case that is the recommended setting)
External monitor set at 100% display scaling.
External monitor set as the main monitor.
Launch app which shows up on external monitor since it it the main monitor.
Drag the app onto the laptop screen.
The THTMLTreeView items are now overlapping (and a standard TTreeView is perfect)