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fix(lint): flag seek-unsafe CSS transitions #3797

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5d90e3f
fix(lint): flag seek-unsafe CSS transitions
Ftgn-dpA Sep 9, 2026
638e015
Merge branch 'main' into fix/css-transition-lint
Ftgn-dpA Sep 9, 2026
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57 changes: 57 additions & 0 deletions packages/lint/src/rules/core.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -673,6 +673,63 @@ describe("core rules", () => {
expect(finding).toBeUndefined();
});

describe("css_transition_used", () => {
it.each([
["transition", "opacity 0.5s ease"],
["transition-duration", "0.5s"],
["transition-delay", "100ms"],
["transition-property", "opacity"],
["-webkit-transition", "opacity 0.5s ease"],
["-webkit-transition-duration", "0.5s"],
])("errors for %s declarations in style blocks", async (property, value) => {
const result = await lintHyperframeHtml(
compositionWithBodyPrefix(`<style>.card { ${property}: ${value}; }</style>`),
);
const finding = result.findings.find((item) => item.code === "css_transition_used");

expect(finding).toMatchObject({ severity: "error", selector: ".card" });
expect(finding?.message).toContain(property);
expect(finding?.fixHint).toContain("paused GSAP timeline");
});

it("errors for an inline transition and identifies its element", async () => {
const result = await lintHyperframeHtml(
compositionWithBodyPrefix(
"",
'<div id="card" style="transition: opacity 0.5s ease"></div>',
),
);
const finding = result.findings.find((item) => item.code === "css_transition_used");

expect(finding).toMatchObject({ severity: "error", elementId: "card" });
expect(finding?.snippet).toContain('id="card"');
});

it.each([
["transition", "none"],
["transition-property", "none"],
["-webkit-transition", "none"],
["-webkit-transition-property", "NONE"],
])("allows %s: %s", async (property, value) => {
const result = await lintHyperframeHtml(
compositionWithBodyPrefix(`<style>.card { ${property}: ${value} !important; }</style>`),
);

expect(result.findings.find((item) => item.code === "css_transition_used")).toBeUndefined();
});

it("ignores custom properties that contain transition in their name", async () => {
const result = await lintHyperframeHtml(
compositionWithBodyPrefix(
'<div id="card" style="--transition-speed: 0.5s"></div>',
"<style>.card { --transition-easing: ease; opacity: 1; }</style>",
),
);

expect(result.findings.find((item) => item.code === "css_transition_used")).toBeUndefined();
});
});

describe("non_deterministic_code", () => {
it("gives randomness guidance for crypto and clock guidance for wall time", async () => {
const result = await lintHyperframeHtml(`<html><body>
Expand Down
80 changes: 80 additions & 0 deletions packages/lint/src/rules/core.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -144,6 +144,37 @@ function ruleForcesOpacityZero(rule: postcss.Rule): boolean {
return forcesOpacityZero;
}

const CSS_TRANSITION_PROPERTY_PATTERN = /^(?:-webkit-)?transition(?:-[a-z][a-z-]*)?$/i;

function isSeekUnsafeTransition(declaration: postcss.Declaration): boolean {
const property = declaration.prop.trim().toLowerCase();
if (property.startsWith("--") || !CSS_TRANSITION_PROPERTY_PATTERN.test(property)) return false;

const value = declaration.value.trim().toLowerCase();
const disablesTransition =
[
"transition",
"-webkit-transition",
"transition-property",
"-webkit-transition-property",
].includes(property) && value === "none";
return !disablesTransition;
}

function cssTransitionFinding(
declaration: postcss.Declaration,
details: Pick<HyperframeLintFinding, "selector" | "elementId" | "snippet">,
): HyperframeLintFinding {
return {
code: "css_transition_used",
severity: "error",
message: `CSS declaration \`${declaration.prop}: ${declaration.value}\` runs on the browser clock and cannot be seeked deterministically across render workers.`,
fixHint:
"Keep the class or attribute swap for state; put the visual change on the paused GSAP timeline.",
...details,
};
}

function isStudioTimelineElement(tag: { raw: string; name: string }): boolean {
if (["script", "style", "link", "meta", "template", "noscript"].includes(tag.name)) {
return false;
Expand Down Expand Up @@ -380,6 +411,55 @@ export const coreRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> = [
return findings;
},

// css_transition_used
({ styles, tags }) => {
const findings: HyperframeLintFinding[] = [];

for (const style of styles) {
if (!/transition/i.test(style.content)) continue;
let root: postcss.Root;
try {
root = postcss.parse(style.content);
} catch {
// The CSS syntax rule reports malformed style blocks separately.
continue;
}
root.walkDecls((declaration) => {
if (!isSeekUnsafeTransition(declaration)) return;
const selector =
declaration.parent?.type === "rule" ? declaration.parent.selector : undefined;
findings.push(
cssTransitionFinding(declaration, {
selector,
snippet: truncateSnippet(declaration.toString()),
}),
);
});
}

for (const tag of tags) {
const inlineStyle = readDecodedAttr(tag.raw, "style");
if (!inlineStyle || !/transition/i.test(inlineStyle)) continue;
let root: postcss.Root;
try {
root = postcss.parse(inlineStyle);
} catch {
continue;
}
root.walkDecls((declaration) => {
if (!isSeekUnsafeTransition(declaration)) return;
findings.push(
cssTransitionFinding(declaration, {
elementId: readDecodedAttr(tag.raw, "id") || undefined,
snippet: truncateSnippet(tag.raw),
}),
);
});
}

return findings;
},

// CSS selector safety
({ styles }) => {
const findings: HyperframeLintFinding[] = [];
Expand Down