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diff --git a/image/test/mochitest/test_discardFinishedAnimatedImage.html b/image/test/mochitest/test_discardFinishedAnimatedImage.html
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index 0000000000..190cb1d1a0
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+++ b/image/test/mochitest/test_discardFinishedAnimatedImage.html
@@ -0,0 +1,144 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <title>Test that img.decode works on finished, discarded animated images</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script src="/tests/SimpleTest/WindowSnapshot.js"></script>
+  <script type="text/javascript" src="imgutils.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1629490">Mozilla Bug 1629490</a>
+<div id="container">
+  <img id="finitepng" src="finite-apng.png">
+</div>
+<script class="testbody" type="text/javascript">
+
+SimpleTest.waitForExplicitFinish();
+
+window.onload = runTest;
+
+let discardCallback = undefined;
+let frameUpdateCallback = undefined;
+
+async function runTest() {
+  const kUsingWebRender = SpecialPowers.DOMWindowUtils.layerManagerType.startsWith("WebRender");
+
+  let img = document.getElementById("finitepng");
+
+  await img.decode();
+
+  while (!isItGreen(img)) {
+    // We hit an optimized path in WebRender that doesn't cause a repaint on the
+    // main thread and doesn't seem to send MozAfterPaints.
+    //
+    // https://searchfox.org/mozilla-central/rev/b7f3977978922d44c7d92ae01c0d4cc2baca7bc2/layout/style/ImageLoader.cpp#553
+    await new Promise(resolve => {
+      if (kUsingWebRender) {
+        requestAnimationFrame(() => {
+          requestAnimationFrame(resolve);
+        });
+      } else {
+        window.addEventListener("MozAfterPaint", resolve, { once: true });
+      }
+    });
+  }
+
+  addCallbacks(img);
+
+  let iterationsLeft = 26;
+  while (iterationsLeft > 0) {
+
+    let discardPromise = new Promise(resolve => {
+        discardCallback = resolve;
+    });
+
+    document.getElementById("container").style.display = "none";
+    document.documentElement.offsetLeft; // force that style to take effect
+    requestDiscard(img);
+
+    await new Promise(resolve => {requestAnimationFrame(() => { requestAnimationFrame(resolve); }); });
+
+    await discardPromise;
+    await new Promise(resolve => {requestAnimationFrame(() => { requestAnimationFrame(resolve); }); });
+
+    let waitForFrameUpdate = new Promise(resolve => {
+        frameUpdateCallback = resolve;
+    });
+
+    document.getElementById("container").style.display = "";
+    document.documentElement.offsetLeft; // force that style to take effect
+
+    await img.decode();
+
+    await new Promise(resolve => requestAnimationFrame(resolve));
+
+    await waitForFrameUpdate;
+
+    ok(isItGreen(img), "should be green");
+
+    iterationsLeft--;
+    await new Promise(resolve => {requestAnimationFrame(() => { requestAnimationFrame(resolve); }); });
+
+  }
+
+  removeObserver(img);
+  SimpleTest.finish();
+}
+
+function isItGreen(img) {
+  let rect = img.getBoundingClientRect();
+  let r = {left: rect.left + 5, top: rect.top + 5, width: 5, height: 5};
+  let c = SpecialPowers.snapshotWindowWithOptions(window, r);
+  let d = c.getContext('2d').getImageData(0,0,5,5).data;
+  let isGreen = true;
+  for (let i = 0; i < 5*5; i++) {
+    if (d[4*i] != 0 || d[4*i + 1] != 128 || d[4*i + 2] != 0 || d[4*i + 3] != 255) {
+      isGreen = false;
+    }
+  }
+  return isGreen;
+}
+
+
+let scriptedObserver = undefined;
+let imgLoadingContent = undefined;
+function addCallbacks(anImage) {
+  var observer = new ImageDecoderObserverStub();
+  observer.discard = function () {
+    if (discardCallback != undefined) {
+      let localDiscardCallback = discardCallback;
+      discardCallback = undefined;
+      setTimeout(localDiscardCallback, 0);
+    }
+  };
+  observer.frameUpdate = function () {
+    if (frameUpdateCallback != undefined) {
+      let localFrameUpdateCallback = frameUpdateCallback;
+      frameUpdateCallback = undefined;
+      setTimeout(localFrameUpdateCallback, 0);
+    }
+  };
+  observer = SpecialPowers.wrapCallbackObject(observer);
+
+  scriptedObserver = SpecialPowers.Cc["@mozilla.org/image/tools;1"]
+                           .getService(SpecialPowers.Ci.imgITools)
+                           .createScriptedObserver(observer);
+
+  imgLoadingContent = SpecialPowers.wrap(anImage);
+  imgLoadingContent.addObserver(scriptedObserver);
+}
+
+function removeObserver(anImage) {
+  imgLoadingContent.removeObserver(scriptedObserver);
+}
+
+function requestDiscard(anImage) {
+  var request = SpecialPowers.wrap(anImage)
+      .getRequest(SpecialPowers.Ci.nsIImageLoadingContent.CURRENT_REQUEST);
+  setTimeout(() => request.requestDiscard(), 0);
+}
+
+</script>
+</body>
+</html>