A tile boundary that lands exactly on an output pixel centre puts the tile
fraction at exactly 0 or 1, and the sample position at the source's geometric
edge — where sample_bilinear mixes in half a texel of the transparent outside.
The fold column comes out half-faded: a visible seam line along the mirrored
boundary, which sweeps the tile as Tile Center animates across pixel centres
(#2820). Both the CPU path and the GPU kernel switch to the clamped sampler,
which repeats the edge texel there; interior samples are untouched, and the
default single-tile identity mapping already sampled texel centres.
The regression test puts the folds exactly on pixel centres (tileWidth 50%,
centre on the layer, mirror on) and asserts the fold columns stay opaque and
equal the edge texels they mirror — it fails on the unfixed code at alpha 0.5.
Fixes#2820
Co-authored-by: rahul05ranjan <11793455+rahul05ranjan@users.noreply.github.com>
rahul05ranjan
540ee9fbc5