Positioning of render_source plane masters was incorrect in some cases.
BYOND Version:515.1614
Operating System:Windows 11 Pro 64-bit
Web Browser:Chrome
Applies to:Dream Seeker
Status: Resolved (515.1615)

This issue has been resolved.
Descriptive Problem Summary:

When targeting a render_source with an alpha mask that's the product of a plane_master, I'm getting strange positioning results of the resultant alpha mask. This also seems to affect composite filters. The expected behavior is that the slate will appear on screen in the same position as it would have rendered as a part of the master plane. The outcome though, is that the slate is being cropped to the extent of the render area, and then positioned at the top-left of the screen when used on a non-plane master screen object, which is really, really strange behavior.

A test case for this was prepared in BYONDiscord, but as it uses art assets from another developer I was debugging this issue for, and designing a vfx render stack for, I will not forward it here.
I'm unsure how much of this is actually a bug, or a result of render_target/source having intractable, unresolved feature gaps.

It may be worthwhile to talk about some sort of way to pass size-aware anchor point information to slates in order to allow the developer to inform the client where slates are expected to actually draw, be they anchored at the bottom-left of the object, the center of the screen, etc. The fundamental problem is that we have no way to know what the size of a slate is going to wind up being according to the engine, and we have no way to tell the engine to take its width/height into account, or target anchor points at all. It feels like we keep finding situations in which the engine interprets developer intention wrong, and I worry that we're repeatedly getting these issues "fixed" while unfixing other cases without being fully aware of it.
Lummox JR resolved issue with message:
Positioning of render_source plane masters was incorrect in some cases.