Bug 1564649 - Reflow all columns when a multicol with non-auto block-size balances its columns at a different block-size. r=TYLin

Depends on D37516

Differential Revision: https://phabricator.services.mozilla.com/D37517

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

layout/generic/nsColumnSetFrame.cpp

@@ -598,6 +598,11 @@ nsColumnSetFrame::ColumnBalanceData nsColumnSetFrame::ReflowChildren(
   // will be adjusted later, after our correct contentSize is known.
   nsSize containerSize = aReflowInput.ComputedSizeAsContainerIfConstrained();
 
+  LogicalSize computedSize =
+      StaticPrefs::layout_css_column_span_enabled()
+          ? aReflowInput.mCBReflowInput->ComputedSize(wm)
+          : aReflowInput.ComputedSize(wm);
+
   // For RTL, since the columns might not fill the frame exactly, we
   // need to account for the slop. Otherwise we'll waste time moving the
   // columns by some tiny amount
@@ -641,8 +646,16 @@ nsColumnSetFrame::ColumnBalanceData nsColumnSetFrame::ReflowChildren(
                            !NS_SUBTREE_DIRTY(child->GetNextSibling());
     // If column-fill is auto (not the default), then we might need to
     // move content between columns for any change in column block-size.
+    //
+    // The same is true if we have a non-'auto' computed block-size.
+    //
+    // FIXME: It's not clear to me why it's *ever* valid to have
+    // skipIncremental be true when changingBSize is true, since it
+    // seems like a child broken over multiple columns might need to
+    // change the size of the fragment in each column.
     if (skipIncremental && changingBSize &&
-        StyleColumn()->mColumnFill == StyleColumnFill::Auto) {
+        (StyleColumn()->mColumnFill == StyleColumnFill::Auto ||
+         computedSize.BSize(wm) != NS_UNCONSTRAINEDSIZE)) {
       skipIncremental = false;
     }
     // If we need to pull up content from the prev-in-flow then this is not just
@@ -706,11 +719,6 @@ nsColumnSetFrame::ColumnBalanceData nsColumnSetFrame::ReflowChildren(
         availSize.BSize(wm) = GetAvailableContentBSize(aReflowInput);
       }
 
-      LogicalSize computedSize =
-          StaticPrefs::layout_css_column_span_enabled()
-              ? aReflowInput.mCBReflowInput->ComputedSize(wm)
-              : aReflowInput.ComputedSize(wm);
-
       if (reflowNext) {
         child->MarkSubtreeDirty();
       }

layout/reftests/columns/reftest.list

@@ -42,7 +42,7 @@ fuzzy-if(browserIsRemote&&winWidget,0-142,0-276) == fieldset-columns-001.html fi
 == break-avoid-line-position-1.html break-avoid-line-position-1-ref.html
 pref(layout.css.column-span.enabled,true) == column-span-bidi-1.html column-span-bidi-1-ref.html
 pref(layout.css.column-span.enabled,true) == first-line-first-letter.html first-line-first-letter-ref.html
-fails == zero-height-nondirty-reflow.html zero-height-nondirty-reflow-ref.html
+== zero-height-nondirty-reflow.html zero-height-nondirty-reflow-ref.html
 
 # The following lines are duplicates of other lines from further up in this
 # manifest. They're listed again here so we can re-run these tests with