--line mode=edge issues with Compositing Window Managers

[daltomi]

These are some problems known in CWM at the time of using the area selection: scrot --select --line mode=edge
Note that scrot --select --freeze works well in them.

MATE desktop
It only draws a part of the rectangle, at the beginning. Notice that the CWM is by software.

xcompmgr
Draw the shadow. Beyond edge has the dock window property and is indicated to xcompmgr that does not draw shadows (-C) in that type of windows, it does it anyway.

[phord]

I tried adding an XUnmapWindow() just before XShapeCombineRectangles(). This does have the effect of clearing the dropped rectangle, and it sometimes (but not always) causes the new rectangle to be drawn correctly.

I suspect it might be safer/saner to draw on a transparent window, but I understand this requires a compositor. But I know almost nothing about the X apis, so I could be way off.