--- a/widget/headless/HeadlessWidget.cpp
+++ b/widget/headless/HeadlessWidget.cpp
@@ -156,18 +156,21 @@ HeadlessWidget::GetLayerManager(PLayerTr
   return mLayerManager;
 }
 
 void
 HeadlessWidget::Resize(double aWidth,
                        double aHeight,
                        bool   aRepaint)
 {
-  mBounds.SizeTo(LayoutDeviceIntSize(NSToIntRound(aWidth),
-                                     NSToIntRound(aHeight)));
+  int32_t width = NSToIntRound(aWidth);
+  int32_t height = NSToIntRound(aHeight);
+  ConstrainSize(&width, &height);
+  mBounds.SizeTo(LayoutDeviceIntSize(width, height));
+
   if (mLayerManager) {
     RefPtr<gfxContext> ctx = CreateDefaultTarget(IntSize(mBounds.width, mBounds.height));
     mLayerManager->AsBasicLayerManager()->SetDefaultTarget(ctx);
   }
   if (mWidgetListener) {
     mWidgetListener->WindowResized(this, mBounds.width, mBounds.height);
   }
   if (mAttachedWidgetListener) {