diff --git a/widget/cocoa/nsChildView.mm b/widget/cocoa/nsChildView.mm
index 1832ca4aedff..442cc054dc9a 100644
--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -948,6 +948,9 @@ nsChildView::BackingScaleFactorChanged()
     if (presShell) {
       presShell->BackingScaleFactorChanged();
     }
+    // When the backing scale factor changes, so does our size in device pixels
+    // (though not in display pixels).
+    ReportSizeEvent();
   }
 }
 
diff --git a/widget/cocoa/nsCocoaWindow.mm b/widget/cocoa/nsCocoaWindow.mm
index 98ab7b21173d..d1d4682b55aa 100644
--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -1704,6 +1704,9 @@ nsCocoaWindow::BackingScaleFactorChanged()
   if (presShell) {
     presShell->BackingScaleFactorChanged();
   }
+  // When the backing scale factor changes, so does our size in device pixels
+  // (though not in display pixels).
+  ReportSizeEvent();
 }
 
 int32_t