Created attachment 131874 [details]
Two top docks of the same thickness show a size difference when one has margins-screen set to --- and the other to 0 or more.

SUMMARY

The option "Hide floating gap for maximised windows" does not fully annull the effect of the setting "margins: screen". Instead, a difference of a few pixels is noticeable.

STEPS TO REPRODUCE
1. Create a dock, for instance a top bar occupying half of the screen
2. Set the dock to "windows go below" for ease of measurement
3. Enable "Hide floating gap for maximised windows"
4. Set Margins-screen to any value above ---, including 0; 
5. Open a maximised window, then change the margins-screen value to ---. The dock now shrinks slightly, both in width and in height.

EXPECTED RESULT
The option "Hide floating gap for maximised windows" should annull the margin effect when windows are maximised, leaving the dock exactly as  thick as if the margin setting were ---. Instead it leaves an additional thickness, as shown in the picture.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: OpenSuse Tumbleweed

KDE Plasma Version: 15.19.5
KDE Frameworks Version: 5.73.0
Qt Version: 5.15.1

ADDITIONAL INFORMATION

The attached image shows two top docks of the same thickness: the left one has its margins-screen set to ---, the right one set to 5 (any value leads to the same effect). The left dock is exactly as thick as the firefox bar while the right one is slightly thicker