1 files changed, 88 insertions, 0 deletions
diff --git a/layout/base/tests/browser_bug1787079.js b/layout/base/tests/browser_bug1787079.js
new file mode 100644
index 0000000000..2161b1f3dc
--- /dev/null
+++ b/layout/base/tests/browser_bug1787079.js
@@ -0,0 +1,88 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const PAGECONTENT =
+  "<!DOCTYPE html>" +
+  "<html>" +
+  "<style>" +
+  "html { height: 100vh; }" +
+  "</style>" +
+  "</html>";
+
+const pageUrl = "data:text/html," + encodeURIComponent(PAGECONTENT);
+
+add_task(async function test() {
+  SpecialPowers.DOMWindowUtils.setHiDPIMode(true);
+  registerCleanupFunction(() => {
+    SpecialPowers.DOMWindowUtils.restoreHiDPIMode();
+  });
+
+  const tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, pageUrl);
+
+  // Enter fullscreen.
+  let fullscreenChangePromise = BrowserTestUtils.waitForContentEvent(
+    tab.linkedBrowser,
+    "fullscreenchange"
+  );
+  await SpecialPowers.spawn(tab.linkedBrowser, [], () => {
+    content.document.documentElement.requestFullscreen();
+  });
+  await fullscreenChangePromise;
+
+  let [originalInnerWidth, originalInnerHeight] = await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [],
+    () => {
+      return [content.window.innerWidth, content.window.innerHeight];
+    }
+  );
+
+  // Then change the DPI.
+  let originalPixelRatio = await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [],
+    () => {
+      return content.window.devicePixelRatio;
+    }
+  );
+  let dpiChangedPromise = TestUtils.waitForCondition(async () => {
+    let pixelRatio = await SpecialPowers.spawn(tab.linkedBrowser, [], () => {
+      return content.window.devicePixelRatio;
+    });
+    return pixelRatio != originalPixelRatio;
+  }, "Make sure the DPI changed");
+  SpecialPowers.DOMWindowUtils.setHiDPIMode(false);
+  await dpiChangedPromise;
+
+  let [innerWidth, innerHeight] = await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [],
+    () => {
+      return [content.window.innerWidth, content.window.innerHeight];
+    }
+  );
+
+  Assert.less(
+    originalInnerWidth,
+    innerWidth,
+    "window.innerWidth on a lower DPI should be greater than the original"
+  );
+  Assert.less(
+    originalInnerHeight,
+    innerHeight,
+    "window.innerHeight on a lower DPI should be greater than the original"
+  );
+
+  fullscreenChangePromise = BrowserTestUtils.waitForContentEvent(
+    tab.linkedBrowser,
+    "fullscreenchange"
+  );
+  await SpecialPowers.spawn(tab.linkedBrowser, [], () => {
+    content.document.exitFullscreen();
+  });
+  await fullscreenChangePromise;
+
+  BrowserTestUtils.removeTab(tab);
+});