I’ve been dealing with this issue for a few months now (on v8.6.2), and found this old thread where people mentioned the same thing.

Here’s the original repro steps, which I will add to:

This bug requires Dual Monitor setup
Notepad is maximized on the SECONDARY display. --It works fine on the primary display
When viewing Notepad++ on the second display you’ll notice a 1 or 2 pixel top bar that’s missing (like notepad++ isn’t really using the full display.)
Move the mouse to the tip top and click— the window behind is grabbed instead.

Here’s the extra thing: I think this issue only started when I changed from a dual 1080p monitor setup to a setup with a 4k primary monitor and a 1080p secondary monitor. Maybe that’s the missing piece to reproduce this issue.