[desktoppackage] Fix ruler handles' defaultPosition (reset behavior)

Panel length turned out to be not what it claimed to be, which caused a bug to sneak into the panel configuration's ruler handles: they tried to reset to this preferred length instead of current/actual length, and without additional checks that could've ended up wider/taller than the screen, rendering one or both pairs of resize handles unreachable and unusable until a panel is brought to some other edge of a screen.

Amends 085087f8 from !1055 (merged)