1 files changed, 29 insertions, 0 deletions
diff --git a/testing/web-platform/mozilla/tests/common/reftest-zoom.js b/testing/web-platform/mozilla/tests/common/reftest-zoom.js
new file mode 100644
index 0000000000..a20f2cc800
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+++ b/testing/web-platform/mozilla/tests/common/reftest-zoom.js
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+// This JS file allows to emulate reftest-zoom.
+// See https://firefox-source-docs.mozilla.org/layout/Reftest.html#zoom-tests-reftest-zoom-float
+
+// Retrieve reftest-zoom attribute.
+const reftestZoom = "reftest-zoom";
+const root = document.documentElement;
+if (!root.hasAttribute(reftestZoom)) {
+  throw new Error(`${reftestZoom} attribute not found on the root element.`);
+}
+
+// Parse reftest-zoom value.
+let zoom = parseFloat(root.getAttribute(reftestZoom));
+if (Number.isNaN(zoom)) {
+  throw new Error(`${reftestZoom} is not a float number.`);
+}
+
+// Clamp reftest-zoom value.
+let minZoom = SpecialPowers.getIntPref("zoom.minPercent") / 100;
+let maxZoom = SpecialPowers.getIntPref("zoom.maxPercent") / 100;
+zoom = Math.min(Math.max(zoom, minZoom), maxZoom);
+
+// Ensure the original zoom level is restored after the screenshot.
+const originalZoom = SpecialPowers.getFullZoom(window);
+window.addEventListener("beforeunload", () => {
+  SpecialPowers.setFullZoom(window, originalZoom);
+});
+
+// Set the zoom level to the specified value.
+SpecialPowers.setFullZoom(window, zoom);