SubtreeVisibility: Allow scoped force locks to force the same lock.

This patch allows the existing locks to be forced multiple times by
keeping count of how many times to force them.

R=cbiesinger@chromium.org, chrishtr@chromium.org

Change-Id: Ieccb60f9698edcb5aad2b57792a53f164563fae4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144618Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758835}

third_party/blink/renderer/core/display_lock/display_lock_context.cc

@@ -449,31 +449,33 @@ bool DisplayLockContext::ShouldCommitForActivation(
 
 DisplayLockContext::ScopedForcedUpdate
 DisplayLockContext::GetScopedForcedUpdate() {
-  if (!is_locked_)
-    return ScopedForcedUpdate(nullptr);
-
-  DCHECK(!update_forced_);
-  update_forced_ = true;
-  TRACE_EVENT_NESTABLE_ASYNC_BEGIN0(
-      TRACE_DISABLED_BY_DEFAULT("blink.debug.display_lock"), "LockForced",
-      TRACE_ID_LOCAL(this));
-
-  // Now that the update is forced, we should ensure that style layout, and
-  // prepaint code can reach it via dirty bits. Note that paint isn't a part of
-  // this, since |update_forced_| doesn't force paint to happen. See
-  // ShouldPaint().
-  MarkForStyleRecalcIfNeeded();
-  MarkForLayoutIfNeeded();
-  MarkAncestorsForPrePaintIfNeeded();
+  ++update_forced_;
+  if (update_forced_ == 1) {
+    TRACE_EVENT_NESTABLE_ASYNC_BEGIN0(
+        TRACE_DISABLED_BY_DEFAULT("blink.debug.display_lock"), "LockForced",
+        TRACE_ID_LOCAL(this));
+  }
+
+  if (IsLocked()) {
+    // Now that the update is forced, we should ensure that style layout, and
+    // prepaint code can reach it via dirty bits. Note that paint isn't a part
+    // of this, since |update_forced_| doesn't force paint to happen. See
+    // ShouldPaint().
+    MarkForStyleRecalcIfNeeded();
+    MarkForLayoutIfNeeded();
+    MarkAncestorsForPrePaintIfNeeded();
+  }
   return ScopedForcedUpdate(this);
 }
 
 void DisplayLockContext::NotifyForcedUpdateScopeEnded() {
   DCHECK(update_forced_);
-  update_forced_ = false;
-  TRACE_EVENT_NESTABLE_ASYNC_END0(
-      TRACE_DISABLED_BY_DEFAULT("blink.debug.display_lock"), "LockForced",
-      TRACE_ID_LOCAL(this));
+  --update_forced_;
+  if (update_forced_ == 0) {
+    TRACE_EVENT_NESTABLE_ASYNC_END0(
+        TRACE_DISABLED_BY_DEFAULT("blink.debug.display_lock"), "LockForced",
+        TRACE_ID_LOCAL(this));
+  }
 }
 
 void DisplayLockContext::Unlock() {
@@ -605,7 +607,7 @@ bool DisplayLockContext::MarkForLayoutIfNeeded() {
   if (IsElementDirtyForLayout()) {
     // Forces the marking of ancestors to happen, even if
     // |DisplayLockContext::ShouldLayout()| returns false.
-    base::AutoReset<bool> scoped_force(&update_forced_, true);
+    base::AutoReset<int> scoped_force(&update_forced_, update_forced_ + 1);
     if (child_layout_was_blocked_) {
       // We've previously blocked a child traversal when doing self-layout for
       // the locked element, so we're marking it with child-needs-layout so that

third_party/blink/renderer/core/display_lock/display_lock_context.h

@@ -328,7 +328,8 @@ class CORE_EXPORT DisplayLockContext final
   // style recalc on them.
   HeapHashSet<Member<Element>> whitespace_reattach_set_;
 
-  bool update_forced_ = false;
+  // If non-zero, then the update has been forced.
+  int update_forced_ = 0;
 
   StyleType blocked_style_traversal_type_ = kStyleUpdateNotRequired;
   // Signifies whether we've blocked a layout tree reattachment on |element_|'s

third_party/blink/renderer/core/display_lock/display_lock_utilities.cc

@@ -51,8 +51,6 @@ bool UpdateStyleAndLayoutForRangeIfNeeded(const EphemeralRangeInFlatTree& range,
                                                                    reason)) {
       DCHECK(locked_activatable_ancestor->GetDisplayLockContext());
       DCHECK(locked_activatable_ancestor->GetDisplayLockContext()->IsLocked());
-      DCHECK(!locked_activatable_ancestor->GetDisplayLockContext()
-                  ->UpdateForced());
       scoped_forced_update_list_.push_back(
           locked_activatable_ancestor->GetDisplayLockContext()
               ->GetScopedForcedUpdate());
@@ -212,10 +210,8 @@ DisplayLockUtilities::ScopedChainForcedUpdate::ScopedChainForcedUpdate(
     auto* ancestor_node = DynamicTo<Element>(ancestor);
     if (!ancestor_node)
       continue;
-    if (auto* context = ancestor_node->GetDisplayLockContext()) {
-      DCHECK(!context->UpdateForced());
+    if (auto* context = ancestor_node->GetDisplayLockContext())
       scoped_update_forced_list_.push_back(context->GetScopedForcedUpdate());
-    }
   }
 }