Add Placement bit without removing others

When scheduling a Placement effect, we should add the Placement bit without resetting the others. In the old fork, there are no flags to reset, anyway, since by the time we reach the child reconciler, the flags will have already been reset. However, in the effects refactor, "static" flags are not reset, so this can actually manifest as a bug. See #20285 for a regression test.
facebook · Dec 7, 2020 · a040878 · a040878
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commit a040878
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diff --git a/packages/react-reconciler/src/ReactChildFiber.new.js b/packages/react-reconciler/src/ReactChildFiber.new.js
@@ -342,15 +342,15 @@ function ChildReconciler(shouldTrackSideEffects) {
       const oldIndex = current.index;
       if (oldIndex < lastPlacedIndex) {
         // This is a move.
-        newFiber.flags = Placement;
+        newFiber.flags |= Placement;
         return lastPlacedIndex;
       } else {
         // This item can stay in place.
         return oldIndex;
       }
     } else {
       // This is an insertion.
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
       return lastPlacedIndex;
     }
   }
@@ -359,7 +359,7 @@ function ChildReconciler(shouldTrackSideEffects) {
     // This is simpler for the single child case. We only need to do a
     // placement for inserting new children.
     if (shouldTrackSideEffects && newFiber.alternate === null) {
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
     }
     return newFiber;
   }

diff --git a/packages/react-reconciler/src/ReactChildFiber.old.js b/packages/react-reconciler/src/ReactChildFiber.old.js
@@ -342,15 +342,15 @@ function ChildReconciler(shouldTrackSideEffects) {
       const oldIndex = current.index;
       if (oldIndex < lastPlacedIndex) {
         // This is a move.
-        newFiber.flags = Placement;
+        newFiber.flags |= Placement;
         return lastPlacedIndex;
       } else {
         // This item can stay in place.
         return oldIndex;
       }
     } else {
       // This is an insertion.
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
       return lastPlacedIndex;
     }
   }
@@ -359,7 +359,7 @@ function ChildReconciler(shouldTrackSideEffects) {
     // This is simpler for the single child case. We only need to do a
     // placement for inserting new children.
     if (shouldTrackSideEffects && newFiber.alternate === null) {
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
     }
     return newFiber;
   }