[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.