--- a/gfx/layers/apz/src/InputBlockState.h
+++ b/gfx/layers/apz/src/InputBlockState.h
@@ -37,17 +37,17 @@ class KeyboardBlockState;
  */
 class InputBlockState : public RefCounted<InputBlockState>
 {
 public:
   MOZ_DECLARE_REFCOUNTED_TYPENAME(InputBlockState)
 
   static const uint64_t NO_BLOCK_ID = 0;
 
-  enum class TargetConfirmationState {
+  enum class TargetConfirmationState : uint8_t {
     eUnconfirmed,
     eTimedOut,
     eTimedOutAndMainThreadResponded,
     eConfirmed
   };
 
   explicit InputBlockState(const RefPtr<AsyncPanZoomController>& aTargetApzc,
                            TargetConfirmationFlags aFlags);