(WIP) Paint invalidation for slimming paint v2

Source/core/layout/LayoutBox.cpp

Index: Source/core/layout/LayoutBox.cpp
diff --git a/Source/core/layout/LayoutBox.cpp b/Source/core/layout/LayoutBox.cpp
index aff8b7f4339afcf14c19215ae9c50f6b2c768c16..fdc44cfd005baa37afd79a11f20e734f44067648 100644
--- a/Source/core/layout/LayoutBox.cpp
+++ b/Source/core/layout/LayoutBox.cpp
@@ -1400,6 +1400,27 @@ PaintInvalidationReason LayoutBox::invalidatePaintIfNeeded(PaintInvalidationStat
     return reason;
 }
 
+PaintInvalidationReason LayoutBox::invalidatePaintIfNeededForSlimmingPaintV2(const LayoutPoint& paintOffset)
+{
+    PaintInvalidationReason fullInvalidationReason = fullPaintInvalidationReason();
+    // If the current paint invalidation reason is PaintInvalidationDelayedFull, then this paint invalidation can delayed if the
+    // LayoutBox in question is not on-screen. The logic to decide whether this is appropriate exists at the site of the original
+    // paint invalidation that chose PaintInvalidationDelayedFull.
+    if (fullInvalidationReason == PaintInvalidationDelayedFull) {
+        if (!intersectsVisibleViewport())
+            return PaintInvalidationDelayedFull;
+
+        // Reset state back to regular full paint invalidation if the object is onscreen.
+        setShouldDoFullPaintInvalidation(PaintInvalidationFull);
+    }
+
+    PaintInvalidationReason reason = LayoutBoxModelObject::invalidatePaintIfNeededForSlimmingPaintV2(paintOffset);
+
+    // This is for the next invalidatePaintIfNeeded so must be at the end.
+    savePreviousBoxSizesIfNeeded();
+    return reason;
+}
+
 void LayoutBox::clearPaintInvalidationState(const PaintInvalidationState& paintInvalidationState)
 {
     LayoutBoxModelObject::clearPaintInvalidationState(paintInvalidationState);
@@ -3990,6 +4011,7 @@ PaintInvalidationReason LayoutBox::paintInvalidationReason(const LayoutBoxModelO
     return PaintInvalidationIncremental;
 }
 
+// TODO: Remove incremental invalidation for slimming paint v2.
 void LayoutBox::incrementallyInvalidatePaint(const LayoutBoxModelObject& paintInvalidationContainer, const LayoutRect& oldBounds, const LayoutRect& newBounds, const LayoutPoint& positionFromPaintInvalidationBacking)
 {
     LayoutObject::incrementallyInvalidatePaint(paintInvalidationContainer, oldBounds, newBounds, positionFromPaintInvalidationBacking);
@@ -4583,7 +4605,7 @@ bool LayoutBox::needToSavePreviousBoxSizes()
     LayoutSize paintInvalidationSize = previousPaintInvalidationRectSize();
     // Don't save old box sizes if the paint rect is empty because we'll
     // full invalidate once the paint rect becomes non-empty.
-    if (paintInvalidationSize.isEmpty())
+    if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled() && paintInvalidationSize.isEmpty())
         return false;
 
     // We need the old box sizes only when the box has background, decorations, or masks.
@@ -4593,7 +4615,7 @@ bool LayoutBox::needToSavePreviousBoxSizes()
 
     // No need to save old border box size if we can use size of the old paint
     // rect as the old border box size in the next invalidation.
-    if (paintInvalidationSize != size())
+    if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled() && paintInvalidationSize != size())
         return true;
 
     // Background and mask layers can depend on other boxes than border box. See crbug.com/490533