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diff --git a/toolkit/components/thumbnails/test/browser_thumbnails_fullViewport.js b/toolkit/components/thumbnails/test/browser_thumbnails_fullViewport.js
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+++ b/toolkit/components/thumbnails/test/browser_thumbnails_fullViewport.js
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+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const BUILDER_URL = "https://example.com/document-builder.sjs?html=";
+const PAGE_MARKUP = `
+<html>
+  <head>
+    <style>
+      body {
+        background-color: rgb(0, 255, 0);
+        margin: 0;
+      }
+
+      div {
+        background-color: rgb(255, 0, 0);
+        height: 100vh;
+        width: 100vw;
+        margin-top: 100vh;
+      }
+    </style>
+  </head>
+  <body>
+    <div id="bigredblock"></div>
+  </body>
+</html>
+`;
+const PAGE_URL = BUILDER_URL + encodeURI(PAGE_MARKUP);
+
+/**
+ * These tests ensure that it's possible to capture the full viewport of
+ * a browser, and not just the top region.
+ */
+add_task(async function thumbnails_fullViewport_capture() {
+  // Create a tab with a green background.
+  await BrowserTestUtils.withNewTab(
+    {
+      gBrowser,
+      url: PAGE_URL,
+    },
+    async browser => {
+      let canvas = PageThumbs.createCanvas(window);
+      await PageThumbs.captureToCanvas(browser, canvas, {
+        fullViewport: true,
+      });
+
+      // The red region isn't scrolled in yet, so we should get
+      // a green thumbnail.
+      let ctx = canvas.getContext("2d");
+      let [r, g, b] = ctx.getImageData(0, 0, 1, 1).data;
+      Assert.equal(r, 0, "No red channel");
+      Assert.equal(g, 255, "Full green channel");
+      Assert.equal(b, 0, "No blue channel");
+
+      // Now scroll the red region into view.
+      await SpecialPowers.spawn(browser, [], () => {
+        let redblock = content.document.getElementById("bigredblock");
+        redblock.scrollIntoView(true);
+      });
+
+      await PageThumbs.captureToCanvas(browser, canvas, {
+        fullViewport: true,
+      });
+
+      // The captured region should be red.
+      ctx = canvas.getContext("2d");
+      [r, g, b] = ctx.getImageData(0, 0, 1, 1).data;
+      Assert.equal(r, 255, "Full red channel");
+      Assert.equal(g, 0, "No green channel");
+      Assert.equal(b, 0, "No blue channel");
+    }
+  );
+});