SUMMARY
I have two monitors layout - 1920x1080 & 3440x1440. The yakuake window size is correctly calculated by the left-most display.
So if I setup yakuake width to be 100% 
* When the 1920x1080 monitor is on the left, then yakuake window size is 1920 width even when opened on the big monitor
* When the 3440x1440 monitor is on the left, yakuake window width is 3440 even when opened on the small monitor (making lot of the window rendered outside of visible area)

STEPS TO REPRODUCE
1. Have dual monitor setup with different resolutions. Ideally with the wider monitor on the left.
2. Set yakuake width to 100% (reproducible also with smaller widths, but with 100% it's most notable)
3. Show yakuake on the rigt-side, smaller monitor

OBSERVED RESULT
Yakuake is wider than the monitor width.

EXPECTED RESULT
Yakuake takes exactly 100% of the monitor width.

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

ADDITIONAL INFORMATION