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test(css): detect prefix windows the compat data discards
The generator keeps a prefixed spelling only while it precedes the unprefixed one, so a browser that lost unprefixed support at an engine change gets no window at all. Opera 15 became Chromium 28 and needed `-webkit-` again after Presto 12.1 had shipped these unprefixed; the test lists those 22 spellings so the gap is a decision rather than an oversight, and fails if it closes or a second browser joins it.
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‎test/CssPrefixWindows.unittest.js‎

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"use strict";
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/** @typedef {{ version_added?: string | boolean | null, version_removed?: string | boolean | null, prefix?: string, alternative_name?: string, flags?: EXPECTED_ANY[] }} BcdSupport */
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/** @typedef {{ __compat?: { support: { [browser: string]: BcdSupport | BcdSupport[] } } }} BcdNode */
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const bcd =
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/** @type {{ css: { properties: { [name: string]: BcdNode } } }} */ (
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/** @type {unknown} */ (require("@mdn/browser-compat-data"))
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);
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const { PREFIXED_PROPERTIES, PREFIX_BROWSERS } = require("../lib/css/data");
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/**
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* The browserslist name each BCD key folds onto, for the browsers the prefix
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* tables carry windows for.
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* @type {Map<string, string[]>}
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*/
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const BCD_TO_BROWSERSLIST = new Map([
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["chrome", ["chrome"]],
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["chrome_android", ["and_chr"]],
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["edge", ["edge"]],
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["firefox", ["firefox"]],
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["firefox_android", ["and_ff"]],
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["ie", ["ie", "ie_mob"]],
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["opera", ["opera"]],
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["opera_android", ["op_mob"]],
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["safari", ["safari"]],
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["safari_ios", ["ios_saf"]],
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["samsunginternet_android", ["samsung"]],
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["webview_android", ["android"]]
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]);
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// The one browser whose engine change took its unprefixed support away again:
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// Presto 12.1 shipped these unprefixed, then Opera 15 became Chromium 28 and
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// needed `-webkit-`. Nothing in BCD marks the discontinuity.
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const ENGINE_SWITCHES = new Map([
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["opera", { at: 15, before: "-o-", after: "-webkit-" }]
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]);
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/**
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* @param {string | boolean | null | undefined} version a BCD version
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* @returns {number | null} it as a number, or `null` when it says nothing
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*/
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const versionOf = (version) => {
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if (version === true) return 0;
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if (typeof version !== "string") return null;
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const parsed = Number.parseFloat(version.replace(/^≤/, ""));
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return Number.isNaN(parsed) ? null : parsed;
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};
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/**
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* The generator keeps a prefixed spelling only while it precedes the unprefixed
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* one. This re-runs that decision to collect what it discarded, so a window the
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* shipped table is missing is visible rather than silent.
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* @returns {{ engineSwitch: string[], sameVersion: number, foreignAlias: number }} what the shipped tables omit, by cause
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*/
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const discardedWindows = () => {
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/** @type {string[]} */
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const engineSwitch = [];
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let sameVersion = 0;
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let foreignAlias = 0;
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for (const [property, spellings] of PREFIXED_PROPERTIES) {
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const compat = bcd.css.properties[property];
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if (compat === undefined || compat.__compat === undefined) continue;
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/** @type {Map<string, Set<string>>} */
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const shipped = new Map();
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for (const [spelling, windows] of spellings) {
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shipped.set(spelling, new Set(windows.map(([browser]) => browser)));
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}
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for (const [key, raw] of Object.entries(compat.__compat.support)) {
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const browsers = BCD_TO_BROWSERSLIST.get(key);
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if (browsers === undefined) continue;
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const entries = Array.isArray(raw) ? raw : [raw];
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let unprefixedFrom = null;
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for (const entry of entries) {
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if (
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!entry ||
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entry.prefix ||
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entry.alternative_name ||
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entry.version_removed ||
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entry.flags
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) {
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continue;
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}
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const added = versionOf(entry.version_added);
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if (
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added !== null &&
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(unprefixedFrom === null || added < unprefixedFrom)
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) {
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unprefixedFrom = added;
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}
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}
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if (unprefixedFrom === null) continue;
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for (const entry of entries) {
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if (!entry || entry.flags || !entry.prefix) continue;
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const prefixedFrom = versionOf(entry.version_added);
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if (prefixedFrom === null || prefixedFrom < unprefixedFrom) continue;
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const spelling = entry.prefix + property;
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for (const browser of browsers) {
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const windows = shipped.get(spelling);
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if (windows !== undefined && windows.has(browser)) continue;
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const change = ENGINE_SWITCHES.get(browser);
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if (prefixedFrom === unprefixedFrom) {
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// BCD records one version for both spellings, so which the
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// engine actually needed is not recoverable from it.
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sameVersion++;
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} else if (
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change !== undefined &&
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unprefixedFrom < change.at &&
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prefixedFrom >= change.at &&
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entry.prefix === change.after
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) {
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engineSwitch.push(
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`${spelling} @ ${browser} (unprefixed ${unprefixedFrom}, prefixed again ${prefixedFrom})`
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);
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} else {
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// A prefix that was never this browser's own, adopted late for
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// web compatibility — Firefox reads `-webkit-` spellings it
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// never needed. Dropping those is right.
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foreignAlias++;
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}
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}
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}
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}
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}
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engineSwitch.sort();
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return { engineSwitch, sameVersion, foreignAlias };
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};
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describe("CSS prefix windows", () => {
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const discarded = discardedWindows();
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it("maps every browser the prefix tables carry windows for", () => {
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const mapped = new Set();
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for (const names of BCD_TO_BROWSERSLIST.values()) {
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for (const name of names) mapped.add(name);
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}
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for (const browser of PREFIX_BROWSERS) {
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expect(mapped.has(browser)).toBe(true);
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}
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});
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// Opera 15/16 read `-webkit-`, so a target of those versions is served only
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// the unprefixed spelling it cannot use. Narrow enough to leave, listed here
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// so it is a decision rather than an oversight — and so closing it, or a
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// second browser joining it, fails this test instead of passing unseen.
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it("has one known gap, and it is Opera's engine change", () => {
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expect(discarded.engineSwitch).toEqual([
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"-webkit-background-clip @ opera (unprefixed 10.5, prefixed again 15)",
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"-webkit-background-origin @ opera (unprefixed 10.5, prefixed again 15)",
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"-webkit-background-size @ opera (unprefixed 10, prefixed again 15)",
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"-webkit-border-bottom-left-radius @ opera (unprefixed 10.5, prefixed again 15)",
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"-webkit-border-bottom-right-radius @ opera (unprefixed 10.5, prefixed again 15)",
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"-webkit-border-top-left-radius @ opera (unprefixed 10.5, prefixed again 15)",
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"-webkit-border-top-right-radius @ opera (unprefixed 10.5, prefixed again 15)",
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"-webkit-column-rule @ opera (unprefixed 11.1, prefixed again 15)",
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"-webkit-column-rule-color @ opera (unprefixed 11.1, prefixed again 15)",
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"-webkit-column-rule-style @ opera (unprefixed 11.1, prefixed again 15)",
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"-webkit-column-rule-width @ opera (unprefixed 11.1, prefixed again 15)",
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"-webkit-column-width @ opera (unprefixed 11.1, prefixed again 15)",
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"-webkit-flex-basis @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-flex-direction @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-flex-flow @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-flex-grow @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-flex-shrink @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-transition @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-transition-delay @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-transition-duration @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-transition-property @ opera (unprefixed 12.1, prefixed again 15)",
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"-webkit-transition-timing-function @ opera (unprefixed 12.1, prefixed again 15)"
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]);
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});
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it("discards the rest for a reason that is not an engine change", () => {
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expect(discarded.sameVersion).toBeGreaterThan(0);
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expect(discarded.foreignAlias).toBeGreaterThan(0);
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});
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});

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