A backdrop was data drawn in the frame's own slot, so `GetRegions` and `GetNumRegions` never saw its pieces, and addons that skin by walking a frame's regions missed them.
In the reference, installing a backdrop creates nine Texture regions on the frame: the background on BACKGROUND, then eight border pieces on BORDER, linked into its region list in that order. A resize re-runs their texcoords and colours, and the colour setters tint the background or the border alone.
The pieces are now real regions, drawn through the region path, so edges draw over another frame's BACKGROUND at the same level. `<Layers>` and `<Backdrop>` load in document order. A region leaving its parent empties any Button, CheckButton, StatusBar, Slider or ColorSelect slot holding it, and a dead layout node now seeds only its dependents.
Tests pin the order, colours, resize, teardown and stock quads.
Fixes#883
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
