Bug#1123989: In GIMP, when choosing lossless export to HEIC with YUV420, you get 4:4:4 in the output

[Emma Mueller]

Package: gimp
Version: 3.0.4-3+deb13u2
I grabbed https://heic.digital/download-sample/shelf-christmas-decoration.heic https://heic.digital/download-sample/shelf-christmas-decoration.heic and checked its parameters: ```sh $ ls -la shelf-christmas-decoration.heic -rw-rw-r-- 1 username username 1443404 2025-12-25 17:02:05.728544690 +0100 shelf-christmas-decoration.heic $ heif-info shelf-christmas-decoration.heic MIME type: image/heic main brand: heic compatible brands: mif1, heic image: 4000x3000 (id=49), primary tiles: 8x6, tile size: 512x512 colorspace: YCbCr, 4:2:0 bit depth: 8 thumbnail: 512x384 color profile: no alpha channel: no depth channel: no metadata: Exif: 852 bytes transformations: none region annotations: none properties: ``` Then I opened this file in GIMP and saved it, placing a checkmark at “Lossless” and choosing YUV420, as shown in the attachment. Then I checked the parameters of the exported file: ```sh $ ls -la shelf-christmas-decoration_exported.heic -rw-rw-r-- 1 username username 6026736 2025-12-25 17:05:04.342442317 +0100 shelf-christmas-decoration_exported.heic $ heif-info shelf-christmas-decoration_exported.heic MIME type: image/heic main brand: heic compatible brands: mif1, heic, miaf image: 4000x3000 (id=1), primary colorspace: YCbCr, 4:4:4 bit depth: 8 color profile: prof alpha channel: no depth channel: no metadata: none transformations: none region annotations: none properties: ``` As you see, the size grew, and the subsamling in the actually exported file 4:4:4 differs from the requested subsampling 4:2:0. This change was silent. If technically possible (which need not be the case for the specific input file) GIMP should honor the user's choices and losslessly export using 4:2:0, otherwise the export dialog should prevent the user from choosing the combination of lossless and YUV420.
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