--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -1410,18 +1410,21 @@ ReflowInput::CalculateHypotheticalPositi
     // like a XUL box, etc.
     // Just use the placeholder's block-offset
     aHypotheticalPos.mBStart = placeholderOffset.B(wm);
   }
 
   // Second, determine the hypothetical box's mIStart.
   // How we determine the hypothetical box depends on whether the element
   // would have been inline-level or block-level
-  if (mStyleDisplay->IsOriginalDisplayInlineOutsideStyle()) {
-    // The placeholder represents the left edge of the hypothetical box
+  if (mStyleDisplay->IsOriginalDisplayInlineOutsideStyle() ||
+      mFlags.mIOffsetsNeedCSSAlign) {
+    // The placeholder represents the IStart edge of the hypothetical box.
+    // (Or if mFlags.mIOffsetsNeedCSSAlign is set, it represents the IStart
+    // edge of the Alignment Container.)
     aHypotheticalPos.mIStart = placeholderOffset.I(wm);
   } else {
     aHypotheticalPos.mIStart = blockIStartContentEdge;
   }
 
   // The current coordinate space is that of the nearest block to the placeholder.
   // Convert to the coordinate space of the absolute containing block
   // One weird thing here is that for fixed-positioned elements we want to do