Don’t know if this section is still active but I have an update.
The option “reset window position settings” seems to have fixed the main problems between the versions.
The problems were:
Video on dual monitor: a detached fullscreen display would reset to wherever the main window was started when stopping or changing files.
Video on single monitor: a detached display window, or when in display view, would resize and center the main window when stopping or changing files.
I should add that I was importing an MC23 library to a fresh install of MC26.
I checked to see if the problems would repeat on a reimport of the MC23 library, so I cleared and imported again. No problems.
One problem remains. Window position still resets (no resizing though) on: play, stop and prev/next.