Created attachment 156454 [details]
cross monitor screenshot, left@2x, right@1.5x

SUMMARY
(This is continued from https://bugs.kde.org/show_bug.cgi?id=460675, but might not be related)

I have two monitors side by side, the left one scaled at 200%, the right one 150%. If Spectacle capture a rectangle region screenshot containing parts from both monitors, the part containing 150% monitor is scaled with artifacts not present on screen.

If the rectangle is entirely within either monitor, the result is the same as on screen.

STEPS TO REPRODUCE
1. Create the above monitor setup.
2. Open a integer scaled window (Dolphin or Gwenview, which doesn't support the new fractional-scale protocol yet), put it across monitor border.
3. Use spectacle to capture a rectangle region containing the window above, across monitor border.
4. Save and see the resulting screenshot without scaling **on a integer scaled monitor** 

OBSERVED RESULT
The right half (from 150% scaled monitor) contains weird artifacts, especially on texts. (See attachment "left@2x, right@1.5x.png")

EXPECTED RESULT
The right half should match as closely as possible to what is seen to the eye, without those artifacts.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 5.27.0
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8

ADDITIONAL INFORMATION
I just realized when experimenting with Spectacle, that in order for the window to maintain its shape across monitors in screenshot, some scaling must be done after capturing both monitors. So the resulting artifacts on the right half is (probably) caused by Spectacle scaling up the 150% half, which is already scaled down from 200% by kwin, to match the 200% half. The only problem is the scaling algorithm that introduced those artifacts.