WIP: Skeleton for paint offset caching

Source/platform/graphics/paint/DisplayItemList.cpp

Index: Source/platform/graphics/paint/DisplayItemList.cpp
diff --git a/Source/platform/graphics/paint/DisplayItemList.cpp b/Source/platform/graphics/paint/DisplayItemList.cpp
index 9c8fad763e60b0b3cbb4dbcea05602878a2bd54c..e01253ee38b06f984dfc4c8e16ee9a1f0d72ed4f 100644
--- a/Source/platform/graphics/paint/DisplayItemList.cpp
+++ b/Source/platform/graphics/paint/DisplayItemList.cpp
@@ -139,6 +139,30 @@ bool DisplayItemList::clientCacheIsValid(DisplayItemClient client) const
     return m_validlyCachedClients.contains(client);
 }
 
+void DisplayItemList::invalidatePaintOffset(DisplayItemClient client)
+{
+    ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
+
+    m_validlyCachedClients.remove(client);
+
+#if ENABLE(ASSERT)
+    m_clientsWithPaintOffsetInvalidations.add(client);
+
+    // Ensure no phases slipped in using the old paint offset which would indicate
+    // different phases used different paint offsets, which should not happen.
+    for (const auto& item : m_newDisplayItems)
+        ASSERT(!item.isCached() || item.client() != client);
+#endif
+}
+
+#if ENABLE(ASSERT)
+bool DisplayItemList::paintOffsetWasInvalidated(DisplayItemClient client) const
+{
+    ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
+    return m_clientsWithPaintOffsetInvalidations.contains(client);
+}
+#endif
+
 size_t DisplayItemList::findMatchingItemFromIndex(const DisplayItem::Id& id, const DisplayItemIndicesByClientMap& displayItemIndicesByClient, const DisplayItems& list)
 {
     DisplayItemIndicesByClientMap::const_iterator it = displayItemIndicesByClient.find(id.client);
@@ -294,6 +318,7 @@ void DisplayItemList::commitNewDisplayItems(DisplayListDiff*)
             ASSERT(!RuntimeEnabledFeatures::slimmingPaintUnderInvalidationCheckingEnabled());
             ASSERT(newDisplayItem.isCached());
             ASSERT(clientCacheIsValid(newDisplayItem.client()));
+            ASSERT(!RuntimeEnabledFeatures::slimmingPaintV2Enabled() || !paintOffsetWasInvalidated(newDisplayItem.client()));
             if (!isSynchronized) {
                 DisplayItems::iterator foundIt = findOutOfOrderCachedItem(currentIt, newDisplayItemId, outOfOrderIndexContext);
 
@@ -324,11 +349,15 @@ void DisplayItemList::commitNewDisplayItems(DisplayListDiff*)
             }
         } else {
 #if ENABLE(ASSERT)
-            if (RuntimeEnabledFeatures::slimmingPaintUnderInvalidationCheckingEnabled())
+            if (RuntimeEnabledFeatures::slimmingPaintUnderInvalidationCheckingEnabled()) {
                 checkCachedDisplayItemIsUnchanged(newDisplayItem, outOfOrderIndexContext.displayItemIndicesByClient);
-            else
-                ASSERT(!newDisplayItem.isDrawing() || newDisplayItem.skippedCache() || !clientCacheIsValid(newDisplayItem.client()));
-#endif // ENABLE(ASSERT)
+            } else {
+                ASSERT(!newDisplayItem.isDrawing()
+                    || newDisplayItem.skippedCache()
+                    || !clientCacheIsValid(newDisplayItem.client())
+                    || (RuntimeEnabledFeatures::slimmingPaintV2Enabled() && paintOffsetWasInvalidated(newDisplayItem.client())));
+            }
+#endif
             updatedList.appendByMoving(*newIt, newIt->derivedSize());
 
             if (isSynchronized)
@@ -345,6 +374,10 @@ void DisplayItemList::commitNewDisplayItems(DisplayListDiff*)
     m_validlyCachedClientsDirty = true;
     m_currentDisplayItems.swap(updatedList);
     m_numCachedItems = 0;
+
+#if ENABLE(ASSERT)
+    m_clientsWithPaintOffsetInvalidations.clear();
+#endif
 }
 
 size_t DisplayItemList::approximateUnsharedMemoryUsage() const