Hello Marks,
We apologize for the inconvenience caused, as per the description above, Adobe Reader window is merging with other open windows, Is that correct?
Please check for any pending updates of Adobe Reader from help>check for updates, reboot the machine after installing any available updates of Adobe Reader and see if this brings any difference.
Also, reset the Preferences of Adobe Reader once and reboot the machine, to reset the Preferences, refer to the steps mentioned in the Adobe article How to reset Acrobat Preference settings to default.
If you still experience any issue, please use Acrobat cleaner tool to remove Adobe Reader Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs
reboot the machine and install the latest version of Adobe Reader from Adobe - Adobe Acrobat Reader DC Distribution
Let us know how it goes and share your findings.
Regards,
Anand Sri.