[brazen1]

The resizing issue after a resolution change began months ago, went away for a while, and now it's back for some users.  I got rid of it by installing an older driver and thought it was a driver induced problem.  Others are reporting that older drivers do it now too.  That's with nVidia.  Now I see AMD is behaving the same.  So, either both companies coincidently introduced the problem or the culprit lies elsewhere.  Since DSP is 17.6 and I'm using 18, KODI shouldn't be the culprit because it wasn't present in older builds.  I'm using Windows 1809 and the problem did not exist for a while so it's not Windows.  I used to be able to create the problem without a player involved so I can't pin it on a player although this latest introduction happens when using MPC-BE only.  I don't know how to diagnose it at this point but it has reared its head again.

Open your GPU control panel.
Open KODI normally in full screen.
Press your Windows key on the keyboard.
Bring your GPU control panel into focus.
Change the resolution to 1080p.  It doesn't have to be 3D but you can change to that as well.  The result will be the same.
If Kodi changed size AND after changing resolution back to 2160p it remains resized, you are affected.

Fwiw, using an MPC external player:  After letting the title run its course and auto exit a player error is produced on top of KODI which is in normal fullscreen.  Pressing enter on the Windows player error allows the resolution change to continue and all is well.  This does not happen when the player is manually closed using alt + F4.  As if that's not enough, certain DVD VIDEO_TS titles are crashing KODI v.18.  No rhyme or reason either.  And lest I forget, external players are not updating watched status at all.  Not even with Estuary.

There hasn't been a nightly in quite a few days now but just noticed a new one released today.  I'll be installing now.