Bug 1614101 - Make sure ColumnSet does not split in the last reflow if ColumnSetWrapper's available block-size is unconstrained. r=dbaron

Checking ColumnSet's available block-size [1] makes no sense nowadays
because ColumnSet is an inner frame under ColumnSetWrapper. We should
check ColumnSetWrapper's available block-size instead.

[1] The comment was also misleading, which said it was checking
ColumnSet's "block-size", but it actually was checking available
block-size.

Differential Revision: https://phabricator.services.mozilla.com/D63385

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

layout/generic/nsColumnSetFrame.cpp

@@ -1141,13 +1141,17 @@ void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
       aConfig.mColMaxBSize = aConfig.mKnownFeasibleBSize;
     }
 
-    // If our block-size is unconstrained, make sure that the last column is
+    // This is our last attempt to reflow. If our column container's available
+    // block-size is unconstrained, make sure that the last column is
     // allowed to have arbitrary block-size here, even though we were
     // balancing. Otherwise we'd have to split, and it's not clear what we'd
     // do with that.
+    const bool forceUnboundedLastColumn =
+        aReflowInput.mParentReflowInput->AvailableBSize() ==
+        NS_UNCONSTRAINEDSIZE;
     MarkPrincipalChildrenDirty(this);
     ReflowColumns(aDesiredSize, aReflowInput, aStatus, aConfig,
-                  availableContentBSize == NS_UNCONSTRAINEDSIZE);
+                  forceUnboundedLastColumn);
   }
 }