SUMMARY
As in subject. Working of Scratchpad makes that KDevelop became almost unusable. 

Usually I work with KDevelop on full screen.
Every time I switch tab with editor then in the middle of the screen appears single window of Scratchpad, or rather its ghost, because shows something like screen buffer under this window. I know this is Scratchpad, because when I click on toolbar with label Scratchpad then it disappears, only I switch to another editor then this ghost window appears covering 1/3 of screen.

I turned off plugin in settings, but this didn't help. Scratchpad still appears. I uncheck Scratchpad in toolbar, but this also didn't help. Finally helped restart of KDevelop.

STEPS TO REPRODUCE
1.  Open several files or project with opened several files
2. Turn on plugin Scratchpad
3. Try to switch from one to another tab editor
4. Move mouse cursor

OBSERVED RESULT
In the middle of screen appears either normal Scratchpad window or its ghost (window buffer under window)

EXPECTED RESULT
Should not appear any window related with plugin when tads with editor are switched.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:  Antergos/KDE
(available in About System)
KDE Plasma Version: 5.21.2
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
Issue also happens in today's version of KDevelopw coming from master branch:
kdevelop-git-5.6.2.r288.g0ac533db1e