ID:2077246
 
Resolved
Setting an overlay's plane to FLOAT_PLANE will make it keep the plane of its parent. When icons and icon_states are added to overlays directly, this is the plane they get.
Applies to:DM Language
Status: Resolved (510.1342)

This issue has been resolved.
The behavior of overlays/underlays having their own static plane defined makes it annoying when the plane of the master object changes. Suppose all the overlays are on plane 0 along with the object, but then we move the object to plane 3 along with a bunch of other objects. The overlays disappear because they don't "follow" the object to the new plane and are now behind other objects.

One solution would be to copy what layers have in FLOAT_LAYER to create FLOAT_PLANE. Anything on this plane used as an overlay/underlay would render on the plane of its parent. This does create the problem of which plane to render the object on when it's alone. Another possibility would be tying that logic not into a special plane but into an appearance flag. Either way images and overlay/underlay created appearances should be this way by default to make things simpler.
This is probably doable, actually. I'll have to take a look at the message format and such to see what room I have with planes, but I believe I made planes 2 bytes almost everywhere for future expansion, so that should give me some options.
Lummox JR resolved issue with message:
Setting an overlay's plane to FLOAT_PLANE will make it keep the plane of its parent. When icons and icon_states are added to overlays directly, this is the plane they get.