--- a/layout/generic/nsVideoFrame.cpp
+++ b/layout/generic/nsVideoFrame.cpp
@@ -316,16 +316,18 @@ nsVideoFrame::Reflow(nsPresContext* aPre
     nsSize oldChildSize = child->GetSize();
 
     if (child->GetContent() == mPosterImage) {
       // Reflow the poster frame.
       nsImageFrame* imageFrame = static_cast<nsImageFrame*>(child);
       ReflowOutput kidDesiredSize(aReflowInput);
       WritingMode wm = imageFrame->GetWritingMode();
       LogicalSize availableSize = aReflowInput.AvailableSize(wm);
+      availableSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
+
       LogicalSize cbSize = aMetrics.Size(aMetrics.GetWritingMode()).
                              ConvertTo(wm, aMetrics.GetWritingMode());
       ReflowInput kidReflowInput(aPresContext,
                                        aReflowInput,
                                        imageFrame,
                                        availableSize,
                                        &cbSize);
 
@@ -335,16 +337,20 @@ nsVideoFrame::Reflow(nsPresContext* aPre
           nsRect(nsPoint(borderPadding.left, borderPadding.top),
                  nsSize(aReflowInput.ComputedWidth(),
                         aReflowInput.ComputedHeight()));
       }
       kidReflowInput.SetComputedWidth(posterRenderRect.width);
       kidReflowInput.SetComputedHeight(posterRenderRect.height);
       ReflowChild(imageFrame, aPresContext, kidDesiredSize, kidReflowInput,
                   posterRenderRect.x, posterRenderRect.y, 0, aStatus);
+      MOZ_ASSERT(aStatus.IsFullyComplete(),
+                 "We gave our child unconstrained available block-size, "
+                 "so it should be complete!");
+
       FinishReflowChild(imageFrame, aPresContext,
                         kidDesiredSize, &kidReflowInput,
                         posterRenderRect.x, posterRenderRect.y, 0);
 
 // Android still uses XUL media controls & hence needs this XUL-friendly
 // custom reflow code. This will go away in bug 1310907.
 #ifdef ANDROID
     } else if (child->GetContent() == mVideoControls) {
@@ -367,16 +373,19 @@ nsVideoFrame::Reflow(nsPresContext* aPre
 
       ReflowInput kidReflowInput(aPresContext,
                                        aReflowInput,
                                        child,
                                        availableSize);
       ReflowOutput kidDesiredSize(kidReflowInput);
       ReflowChild(child, aPresContext, kidDesiredSize, kidReflowInput,
                   borderPadding.left, borderPadding.top, 0, aStatus);
+      MOZ_ASSERT(aStatus.IsFullyComplete(),
+                 "We gave our child unconstrained available block-size, "
+                 "so it should be complete!");
 
       if (child->GetContent() == mVideoControls && isBSizeShrinkWrapping) {
         // Resolve our own BSize based on the controls' size in the same axis.
         contentBoxBSize = myWM.IsOrthogonalTo(wm) ?
           kidDesiredSize.ISize(wm) : kidDesiredSize.BSize(wm);
       }
 
       FinishReflowChild(child, aPresContext,