_shadow_alpha() measured the distance to the window's bounding rectangle,
so WIN11/WIN7/WINXP shadows still had square corners: the rounded corner
notch stayed unshadowed and the outer iso-alpha lines met at a right
angle.
Measure against the rectangle inset by the window radius instead. The
corner arcs now shape both the notch and the outer falloff, while the
straight edges keep the previous values. Square styles pass r = 0 and
are unchanged.
TermiNexus
ebd43ef9da