Media: Fix GD editor leaking WebP-lossless quality into other output formats

[irozum]

WP_Image_Editor_GD::set_quality() checks $this->mime_type (the source image's mime type) instead of the output mime type when deciding whether to apply the WebP-lossless quality sentinel (IMG_WEBP_LOSSLESS, 101). Because of that, converting a lossless WebP source to any other format (AVIF, JPEG, etc.) still gets the quality forced to 101 — a value that is intentionally outside the normal [1,100] range and only valid for GD's WebP lossless encoder. On PHP 8.4+, passing 101 to imageavif()/imagejpeg() throws a ValueError, producing a fatal error, 0-byte sub-sizes, and broken wp media regenerate runs; on older PHP it silently applies the wrong quality to the non-WebP output.

This fixes the mime check to use $this->output_mime_type when it's set (falling back to $this->mime_type, matching the same fallback the parent class already uses in WP_Image_Editor::set_quality()), so the lossless override is only applied when the image is actually being encoded as WebP. It also removes the redundant parent::set_quality( $quality, $dims ) call for the 101 sentinel, since that value is always rejected by the parent's [1,100] validation and its WP_Error return was being silently discarded — the direct $this->quality assignment right below it was already the only thing doing anything.

Added a PHPUnit test (Tests_Image_Editor_GD::test_set_quality_does_not_leak_webp_lossless_quality_into_other_formats) that loads a lossless WebP fixture, saves it as JPEG, and asserts the quality used is the normal JPEG default (82) rather than the leaked WebP-lossless sentinel (101).

Trac ticket: https://core.trac.wordpress.org/ticket/65977

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Root-cause analysis, implementation, tests, and PR description. Reviewed by Igor Rozum.

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