Created attachment 167666 [details]
Double-line on minimize all

SUMMARY
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I noticed this visual bug right after update to Plasma 6 and is still visible on 6.0.2
Basically, when all windows are minimized using "minimize all" button (when manually one by one, the bug is not there), the upper line marking opened window is doubled with space between - see the screenshot.
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STEPS TO REPRODUCE
1. Open few apps.
2. Use "minimize all" (shortcut or panel widget)
3. Move the cursor over the icon of an opened, but minimized app.

OBSERVED RESULT
The upper line is doubled with a space between.

EXPECTED RESULT
No doubled line, just as it was before (the doubled line has no function).


SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
The reason for the upper, double line reveals when trying to take a screenshot with Spectacle (delayed rectangle area). Basically, when desktop becomes darkened, and Spectacle expects to draw the area, I see at the same time floating panel and the defloated panel, with the stripe showed on both versions. However, those double panels are not visible on the screenshot.

So basically, the bug is, that the upper stripe is drawn visibly on the floating and not floating version of a panel. 

I'm sending a screenshot that shows the double line, as I see it on a daily basis, and the photo taken by the cell phone, to show you doubled panels and where the issue probably stems from.

It is possible, that the doubled panel on the pre-screenshot view and the double upper marking stripe are different bugs, because normally I see:

1. Floating panel (since all apps are minimized) with the doubled stripe above it
2. During pre-screenshot, the defloated panel is on the foreground (so lower positioned) and the floating panel is in the background and above, and the doubled stripe is then drawn on the floating version, completely opposite to what is happening during "minimize all", where the doubled line is even higher positioned.