When font size inflation is enabled, horizontal resizes of blocks must cause a full dirty reflow. (Bug 627842, patch 5) r=bzbarsky

This change is sufficient because the places that set mHResize to true
other than InitResizeFlags and nsFrame::BoxReflow aren't a problem
because they're in table code whose goal is to force the reflow to
propagate down to the cell, and once we reach the cell we'll hit the
code we've added here.

layout/generic/nsFrame.cpp

@@ -7568,8 +7568,16 @@ nsFrame::BoxReflow(nsBoxLayoutState&        aState,
     // mLastSize before calling Reflow and then switching it back, but
     // However, mLastSize can also be the size passed to BoxReflow by
     // RefreshSizeCache, so that doesn't really make sense.
-    if (metrics->mLastSize.width != aWidth)
+    if (metrics->mLastSize.width != aWidth) {
       reflowState.mFlags.mHResize = true;
+
+      // When font size inflation is enabled, a horizontal resize
+      // requires a full reflow.  See nsHTMLReflowState::InitResizeFlags
+      // for more details.
+      if (nsLayoutUtils::FontSizeInflationEnabled(aPresContext)) {
+        AddStateBits(NS_FRAME_IS_DIRTY);
+      }
+    }
     if (metrics->mLastSize.height != aHeight)
       reflowState.mFlags.mVResize = true;
 

layout/generic/nsHTMLReflowState.cpp

@@ -365,6 +365,31 @@ nsHTMLReflowState::InitResizeFlags(nsPresContext* aPresContext, nsIAtom* aFrameT
   mFlags.mHResize = !(frame->GetStateBits() & NS_FRAME_IS_DIRTY) &&
                     frame->GetSize().width !=
                       mComputedWidth + mComputedBorderPadding.LeftRight();
+  if (mFlags.mHResize &&
+      nsLayoutUtils::FontSizeInflationEnabled(aPresContext)) {
+    // When font size inflation is enabled, the change in the width of a
+    // block (or anything that returns true in
+    // IsContainerForFontSizeInflation) needs to cause a dirty reflow
+    // since it changes the size of text, line-heights, etc.  This is
+    // relatively similar to a classic case of style change reflow,
+    // except that because inflation doesn't affect the intrinsic sizing
+    // codepath, there's no need to invalidate intrinsic sizes.
+    //
+    // Note that this makes horizontal resizing a good bit more
+    // expensive.  However, font size inflation is targeted at a set of
+    // devices (zoom-and-pan devices) where the main use case for
+    // horizontal resizing needing to be efficient (window resizing) is
+    // not present.  It does still increase the cost of dynamic changes
+    // caused by script where a style or content change in one place
+    // causes a resize in another (e.g., rebalancing a table).
+
+    // FIXME: This isn't so great for the cases where
+    // nsHTMLReflowState::SetComputedWith is called, if the first time
+    // we go through InitResizeFlags we set mHResize to true, and then
+    // the second time we'd set it to false even without the
+    // NS_FRAME_IS_DIRTY bit already set.
+    frame->AddStateBits(NS_FRAME_IS_DIRTY);
+  }
 
   // XXX Should we really need to null check mCBReflowState?  (We do for
   // at least nsBoxFrame).