SUMMARY
When taking a screenshot of "Window under mouse cursor" [doesn't seem to affect "active window"] the screenshot includes an area outside the window, where seemingly some decorations from the plasma theme are included. this is independent from the checkbox of "include title and frame of window", and i feel like it should be neither. also this is independent from compositing being enabled, so it doesn't actually reflect the screen content, as the plasma decorations are disabled when compositing is disabled.

STEPS TO REPRODUCE
1. Have a theme which has, e.g. a shadow around the window
2. select "window under mouse pointer" in Spectacle
3. make a screenshot of a window
4. open screenshot in image viewer (e.g. Gwenview)

OBSERVED RESULT
Screenshot is several pixels larger than the window (including areas that are actually off-screen for maximized windows)

EXPECTED RESULT
Screenshot should only contain actual window content


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: OpenSUSE 15.0
(available in About System)
KDE Plasma Version: 17.12.3
KDE Frameworks Version: 5.45.0
Qt Version: 5.9.4

ADDITIONAL INFORMATION
this seems to be related to ksnapshot bug #348131