Bug 1647520 Part 4 - Stop binary search once the feasible and infeasible block-size is within one device pixel. r=heycam

The gist of this patch is: once the feasible and infeasible block-size
is within one device pixel, we choose nextGuess to be the greatest
multiple of one device pixel below or equal to the feasible block-size.
If this nextGuess is infeasible, we will reflow children one last time
by using the last feasible block-size and done with the balancing.

Differential Revision: https://phabricator.services.mozilla.com/D83732

layout/generic/nsColumnSetFrame.cpp

@@ -996,6 +996,7 @@ void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
   // that can break anywhere (thus foiling the linear decrease-by-one
   // search)
   bool maybeContinuousBreakingDetected = false;
+  bool possibleOptimalBSizeDetected = false;
 
   // This is the extra block-size added to the optimal column block-size
   // estimation which is calculated in the while-loop by dividing
@@ -1008,6 +1009,10 @@ void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
   // last column's available block-size is unbounded.
   bool foundFeasibleBSizeCloserToBest = !aUnboundedLastColumn;
 
+  // Stop the binary search when the difference of the feasible and infeasible
+  // block-size is within this gap. Here we use one device pixel.
+  const int32_t gapToStop = aPresContext->DevPixelsToAppUnits(1);
+
   while (!aPresContext->HasPendingInterrupt()) {
     nscoord lastKnownFeasibleBSize = aConfig.mKnownFeasibleBSize;
 
@@ -1067,6 +1072,14 @@ void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
       break;
     }
 
+    const nscoord gap =
+        aConfig.mKnownFeasibleBSize - aConfig.mKnownInfeasibleBSize;
+    if (gap <= gapToStop && possibleOptimalBSizeDetected) {
+      // We detected a possible optimal block-size in the last iteration. If it
+      // is infeasible, we may need to reflow one more time after this loop.
+      break;
+    }
+
     if (lastKnownFeasibleBSize - aConfig.mKnownFeasibleBSize == 1) {
       // We decreased the feasible block-size by one twip only. This could
       // indicate that there is a continuously breakable child frame
@@ -1074,8 +1087,7 @@ void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
       maybeContinuousBreakingDetected = true;
     }
 
-    nscoord nextGuess =
-        (aConfig.mKnownFeasibleBSize + aConfig.mKnownInfeasibleBSize) / 2;
+    nscoord nextGuess = aConfig.mKnownInfeasibleBSize + gap / 2;
     if (aConfig.mKnownFeasibleBSize - nextGuess < extraBlockSize &&
         !maybeContinuousBreakingDetected) {
       // We're close to our target, so just try shrinking just the
@@ -1097,7 +1109,13 @@ void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
       // want to do an unbounded block-size measuring step. Let's just increase
       // from the infeasible block-size by some reasonable amount.
       nextGuess = aConfig.mKnownInfeasibleBSize * 2 + extraBlockSize;
+    } else if (gap <= gapToStop) {
+      // Floor nextGuess to the greatest multiple of gapToStop below or equal to
+      // mKnownFeasibleBSize.
+      nextGuess = aConfig.mKnownFeasibleBSize / gapToStop * gapToStop;
+      possibleOptimalBSizeDetected = true;
     }
+
     // Don't bother guessing more than our block-size constraint.
     nextGuess = std::min(availableContentBSize, nextGuess);