After opening Acrobat Reader DC (in a windowed mode, not in tabbed), it, for some mysterious reason, switches off the Windows 10 Explorer setting for windows to cast shadows. It results in focused folder windows becoming visually borderless, making it very difficult to work with them.
The problem was described here for Acrobat DC:
https://acrobat.uservoice.com/forums/590923-acrobat-for-windows-and-mac/suggestions/38822896-bug-shadows-disappear-on-windows-10-explorer-wind
but it fully (and even more irritably) applies to Acrobat Reader DC, too.

It seems that for some reason, Acrobat Reader changes (switches off) each time the inherent Windows setting which can be found in System > Visual Effects tab > Show shadows under windows checkbox.

The problem persist already for a long time, please fix it!