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Fix image sizes attribute for constrained layout #443

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Fix image sizes attribute for constrained layout #443

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19 changes: 19 additions & 0 deletions src/avifImageService.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,13 +3,32 @@ import sharp from "sharp";

sharp.concurrency(1);

// Width of the Starlight content column: keep in sync with --sl-content-width in
// src/assets/css/variables.css. Images can never be displayed wider than this.
const CONTENT_WIDTH_REM = 47;
const CONTENT_WIDTH_PX = CONTENT_WIDTH_REM * 16;

export default {
...sharpService,

// Let SVGs skip validation entirely — sharp can't process them
validateOptions(options, config) {
if (options.src?.format !== "svg") {
options.format = "avif";

// Astro's "constrained" layout generates
// sizes="(min-width: <image width>px) <image width>px, 100vw"
// which tells the browser the image may fill the whole viewport. Here it
// never does (the content column is capped), so browsers pick huge
// srcset candidates: the 3840w file on a 1440px laptop at 2x, or the 2048w
// file on a 1080p monitor at 1x. Use the real maximum display width instead.
// Images narrower than the column keep Astro's value, which is already correct.
if (
options["data-astro-image"] === "constrained" &&
options.width > CONTENT_WIDTH_PX
) {
options.sizes = `(min-width: ${CONTENT_WIDTH_REM}rem) ${CONTENT_WIDTH_REM}rem, 100vw`;
}
}
return sharpService.validateOptions(options, config);
},
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