diff --git a/testing/specialpowers/content/SpecialPowersChild.jsm b/testing/specialpowers/content/SpecialPowersChild.jsm
index 45a9ae8fed71..280cb00032eb 100644
--- a/testing/specialpowers/content/SpecialPowersChild.jsm
+++ b/testing/specialpowers/content/SpecialPowersChild.jsm
@@ -2245,12 +2245,18 @@ class SpecialPowersChild extends JSWindowActorChild {
    * we need to wait for the updated data.
    */
   contentTransformsReceived(win) {
-    try {
-      // throw if win is not a remote browser.
-      return win.docShell.browserChild.contentTransformsReceived();
-    } catch (e) {
-      return Promise.resolve();
+    while (win) {
+      try {
+        return win.docShell.browserChild.contentTransformsReceived();
+      } catch (ex) {
+        // browserChild getter throws on non-e10s rather than returning null.
+      }
+      if (win == win.parent) {
+        break;
+      }
+      win = win.parent;
     }
+    return Promise.resolve();
   }
 }
 
diff --git a/toolkit/content/tests/browser/head.js b/toolkit/content/tests/browser/head.js
index 4667b7cfc1d9..6531aae023a0 100644
--- a/toolkit/content/tests/browser/head.js
+++ b/toolkit/content/tests/browser/head.js
@@ -200,6 +200,11 @@ class DateTimeTestHelper {
       });
       bc = bc.browsingContext.children[0];
     }
+    await SpecialPowers.spawn(bc, [], async function() {
+      // Ensure that screen coordinates are ok.
+      await SpecialPowers.contentTransformsReceived(content);
+    });
+
     if (openMethod === "click") {
       await BrowserTestUtils.synthesizeMouseAtCenter("input", {}, bc);
     } else if (openMethod === "showPicker") {