Missing check for GetForceReattachLayoutTree().

The style recalc root may be marked GetForceReattachLayoutTree() without
being marked for NeedsStyleRecalc(). Without that check, the
StyleRecalcRoot code was confused to believe we removed the recalc root
from the flat tree when it wasn't.

Bug: 1031934, 1031521
Change-Id: I1a5be72b44932d31adc7a59da06941ab975206a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1958126
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723293}

third_party/blink/renderer/core/css/style_engine_test.cc

@@ -2539,4 +2539,27 @@ TEST_F(StyleEngineTest, RecalcEnsuredStyleOutsideFlatTreeV0) {
   EXPECT_FALSE(GetDocument().body()->ChildNeedsStyleRecalc());
 }
 
+TEST_F(StyleEngineTest, ForceReattachRecalcRootAttachShadow) {
+  ScopedFlatTreeStyleRecalcForTest scope(true);
+
+  GetDocument().body()->SetInnerHTMLFromString(R"HTML(
+    <div id="reattach"></div><div id="host"><span></span></div>
+  )HTML");
+
+  auto* reattach = GetDocument().getElementById("reattach");
+  auto* host = GetDocument().getElementById("host");
+
+  UpdateAllLifecyclePhases();
+
+  reattach->SetForceReattachLayoutTree();
+  EXPECT_FALSE(reattach->NeedsStyleRecalc());
+  EXPECT_EQ(reattach, GetStyleRecalcRoot());
+
+  // Attaching the shadow root will call RemovedFromFlatTree() on the span child
+  // of the host and mark the host for style recalc . Make sure the recalc root
+  // is correctly updated.
+  host->AttachShadowRootInternal(ShadowRootType::kOpen);
+  EXPECT_EQ(GetDocument().body(), GetStyleRecalcRoot());
+}
+
 }  // namespace blink

third_party/blink/renderer/core/css/style_recalc_root.cc

@@ -131,6 +131,7 @@ void StyleRecalcRoot::RemovedFromFlatTree(const Node& node) {
   // make sure we don't have a recalc root outside the flat tree, which is not
   // allowed with FlatTreeStyleRecalc enabled.
   if (GetRootNode()->NeedsStyleRecalc() ||
+      GetRootNode()->GetForceReattachLayoutTree() ||
       GetRootNode()->ChildNeedsStyleRecalc()) {
     return;
   }