Bug 1663439 - Avoid negative avail bsizes in paginated table reflow. r=TYLin, a=tjr

This causes an assertion when printing the test-case, though it turns
out not to be the cause of the bug in the end. Still seems worth fixing,
and will be tested by the test in the following patch.

Differential Revision: https://phabricator.services.mozilla.com/D90145

layout/tables/nsTableFrame.cpp

@@ -110,6 +110,14 @@ struct TableReflowInput {
       availSize.BSize(wm) = std::max(0, availSize.BSize(wm));
     }
   }
+
+  void ReduceAvailableBSizeBy(WritingMode aWM, nscoord aAmount) {
+    if (availSize.BSize(aWM) == NS_UNCONSTRAINEDSIZE) {
+      return;
+    }
+    availSize.BSize(aWM) -= aAmount;
+    availSize.BSize(aWM) = std::max(0, availSize.BSize(aWM));
+  }
 };
 
 }  // namespace mozilla
@@ -2689,9 +2697,7 @@ void nsTableFrame::PlaceChild(TableReflowInput& aReflowInput,
   aReflowInput.bCoord += aKidDesiredSize.BSize(wm);
 
   // If our bsize is constrained, then update the available bsize
-  if (NS_UNCONSTRAINEDSIZE != aReflowInput.availSize.BSize(wm)) {
-    aReflowInput.availSize.BSize(wm) -= aKidDesiredSize.BSize(wm);
-  }
+  aReflowInput.ReduceAvailableBSizeBy(wm, aKidDesiredSize.BSize(wm));
 }
 
 void nsTableFrame::OrderRowGroups(RowGroupArray& aChildren,
@@ -2956,9 +2962,7 @@ void nsTableFrame::ReflowChildren(TableReflowInput& aReflowInput,
         kidReflowInput.mFlags.mIsTopOfPage = false;
       }
       aReflowInput.bCoord += cellSpacingB;
-      if (NS_UNCONSTRAINEDSIZE != aReflowInput.availSize.BSize(wm)) {
-        aReflowInput.availSize.BSize(wm) -= cellSpacingB;
-      }
+      aReflowInput.ReduceAvailableBSizeBy(wm, cellSpacingB);
       // record the presence of a next in flow, it might get destroyed so we
       // need to reorder the row group array
       bool reorder = false;
@@ -3116,10 +3120,7 @@ void nsTableFrame::ReflowChildren(TableReflowInput& aReflowInput,
       }
       aReflowInput.bCoord += kidRect.BSize(wm);
 
-      // If our bsize is constrained then update the available bsize.
-      if (NS_UNCONSTRAINEDSIZE != aReflowInput.availSize.BSize(wm)) {
-        aReflowInput.availSize.BSize(wm) -= cellSpacingB + kidRect.BSize(wm);
-      }
+      aReflowInput.ReduceAvailableBSizeBy(wm, cellSpacingB + kidRect.BSize(wm));
     }
   }