The merged image carries its transparency in the first extra channel
when the layer count is stored negative, or when a "Saving Merged
Transparency" global block (Mtrn, Mt16, Mt32) is present. Photoshop
saves 32-bit documents the second way: a positive layer count, Mt32,
and a fourth channel named "Transparency" (psd-tools transparentbg).
merged_has_alpha only knew the first, so opening such a file added a
"Transparency" alpha channel to the document, and the oracle compared
it as if it were opaque white (83.7 % of pixels off; now 0.9 %, all on
the shape's edge).
Co-authored-by: Claude Opus 5.5 <[email protected]>
