Bug 1548118 Part 3 - Always mark inner ColumnSetWrapper's children dirty before reflowing it if the outer ColumnSetWrapper is balancing. r=dbaron

ColumnSetFrame always tries to reflow column content regardless of it's
dirtiness. Making ColumnSetWrapperFrame's children dirty can have the
same effect.

Differential Revision: https://phabricator.services.mozilla.com/D29435

--HG--
extra : moz-landing-system : lando

layout/generic/nsBlockFrame.cpp

@@ -3367,6 +3367,15 @@ void nsBlockFrame::ReflowBlockFrame(BlockReflowInput& aState,
       cbSize.emplace(LogicalSize(wm, aState.mReflowInput.ComputedISize(),
                                  cbReflowInput->ComputedBSize())
                          .ConvertTo(frame->GetWritingMode(), wm));
+
+      // If a ColumnSetWrapper is in a balancing column content, it may be
+      // pushed or pulled back and forth between column contents. Always add
+      // NS_FRAME_HAS_DIRTY_CHILDREN bit to it so that its ColumnSet children
+      // can have a chance to reflow under current block size constraint.
+      if (aState.mReflowInput.mFlags.mIsColumnBalancing &&
+          frame->IsColumnSetWrapperFrame()) {
+        frame->AddStateBits(NS_FRAME_HAS_DIRTY_CHILDREN);
+      }
     }
 
     blockReflowInput.emplace(

layout/reftests/columns/reftest.list

@@ -49,5 +49,5 @@ pref(layout.css.column-span.enabled,true) == first-line-first-letter.html first-
 # default-enabled (Bug 1426010).
 default-preferences pref(layout.css.column-span.enabled,true)
 fails == min-width-2.html min-width-2-ref.html # Bug 1548100
-fails == column-balancing-nested-001.html column-balancing-nested-001-ref.html # Bug 1548118
+== column-balancing-nested-001.html column-balancing-nested-001-ref.html
 default-preferences