--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -3082,16 +3082,24 @@ nsBlockFrame::ReflowBlockFrame(nsBlockRe
                                   replacedBlock != nsnull, availSpace);
     
     // Now put the Y coordinate back to the top of the top-margin +
     // clearance, and flow the block.
     aState.mY -= topMargin;
     availSpace.y -= topMargin;
     if (NS_UNCONSTRAINEDSIZE != availSpace.height) {
       availSpace.height += topMargin;
+
+      // When there is a pushed float, clearance could equal
+      // NS_UNCONSTRAINEDSIZE (FIXME: is that really a good idea?), but
+      // we don't want that to change a constrained height to an
+      // unconstrained one.
+      if (NS_UNCONSTRAINEDSIZE == availSpace.height) {
+        --availSpace.height;
+      }
     }
     
     // Reflow the block into the available space
     // construct the html reflow state for the block. ReflowBlock 
     // will initialize it
     nsHTMLReflowState blockHtmlRS(aState.mPresContext, aState.mReflowState, frame, 
                                   nsSize(availSpace.width, availSpace.height));
     blockHtmlRS.mFlags.mHasClearance = aLine->HasClearance();