--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -2477,19 +2477,20 @@ Gecko_GetFontMetrics(RawGeckoPresContext
   // and if so, appends PostTraversalTasks to the current ServoStyleSet
   // to be performed immediately after the traversal is finished.  This
   // works well for starting downloadable font loads, since we don't have
   // those fonts available to get metrics for anyway.  Platform fonts and
   // ArrayBuffer-backed FontFace objects are handled synchronously.
 
   nsPresContext* presContext = const_cast<nsPresContext*>(aPresContext);
   presContext->SetUsesExChUnits(true);
-  RefPtr<nsFontMetrics> fm = nsRuleNode::GetMetricsFor(presContext, aIsVertical,
-                                                       aFont, aFontSize,
-                                                       aUseUserFontSet);
+  RefPtr<nsFontMetrics> fm = nsRuleNode::GetMetricsFor(
+      presContext, aIsVertical, aFont, aFontSize, aUseUserFontSet,
+      nsRuleNode::FlushUserFontSet::No);
+
   ret.mXSize = fm->XHeight();
   gfxFloat zeroWidth = fm->GetThebesFontGroup()->GetFirstValidFont()->
                            GetMetrics(fm->Orientation()).zeroOrAveCharWidth;
   ret.mChSize = ceil(aPresContext->AppUnitsPerDevPixel() * zeroWidth);
   return ret;
 }
 
 int32_t

--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -372,28 +372,30 @@ static inline nscoord ScaleViewportCoord
 }
 
 /* static */
 already_AddRefed<nsFontMetrics>
 nsRuleNode::GetMetricsFor(nsPresContext* aPresContext,
                           bool aIsVertical,
                           const nsStyleFont* aStyleFont,
                           nscoord aFontSize,
-                          bool aUseUserFontSet)
+                          bool aUseUserFontSet,
+                          FlushUserFontSet aFlushUserFontSet)
 {
   nsFont font = aStyleFont->mFont;
   font.size = aFontSize;
   gfxFont::Orientation orientation
     = aIsVertical ? gfxFont::eVertical : gfxFont::eHorizontal;
   nsFontMetrics::Params params;
   params.language = aStyleFont->mLanguage;
   params.explicitLanguage = aStyleFont->mExplicitLanguage;
   params.orientation = orientation;
-  params.userFontSet =
-    aUseUserFontSet ? aPresContext->GetUserFontSet() : nullptr;
+  params.userFontSet = aUseUserFontSet
+    ? aPresContext->GetUserFontSet(aFlushUserFontSet == FlushUserFontSet::Yes)
+    : nullptr;
   params.textPerf = aPresContext->GetTextPerfMetrics();
   return aPresContext->DeviceContext()->GetMetricsFor(font, params);
 }
 
 /* static */
 already_AddRefed<nsFontMetrics>
 nsRuleNode::GetMetricsFor(nsPresContext* aPresContext,
                           nsStyleContext* aStyleContext,
@@ -403,18 +405,19 @@ nsRuleNode::GetMetricsFor(nsPresContext*
 {
   bool isVertical = false;
   if (aStyleContext) {
     WritingMode wm(aStyleContext);
     if (wm.IsVertical() && !wm.IsSideways()) {
       isVertical = true;
     }
   }
-  return nsRuleNode::GetMetricsFor(aPresContext, isVertical, aStyleFont,
-                                   aFontSize, aUseUserFontSet);
+  return nsRuleNode::GetMetricsFor(
+      aPresContext, isVertical, aStyleFont, aFontSize, aUseUserFontSet,
+      FlushUserFontSet::Yes);
 }
 
 /* static */
 void
 nsRuleNode::FixupNoneGeneric(nsFont* aFont,
                              const nsPresContext* aPresContext,
                              uint8_t aGenericFontID,
                              const nsFont* aDefaultVariableFont)

--- a/layout/style/nsRuleNode.h
+++ b/layout/style/nsRuleNode.h
@@ -802,21 +802,27 @@ private:
 public:
   // This is infallible; it will never return nullptr.
   static already_AddRefed<nsRuleNode> CreateRootNode(nsPresContext* aPresContext);
 
   static void EnsureBlockDisplay(mozilla::StyleDisplay& display,
                                  bool aConvertListItem = false);
   static void EnsureInlineDisplay(mozilla::StyleDisplay& display);
 
+  enum class FlushUserFontSet {
+    Yes,
+    No,
+  };
+
   static already_AddRefed<nsFontMetrics> GetMetricsFor(nsPresContext* aPresContext,
                                                        bool aIsVertical,
                                                        const nsStyleFont* aStyleFont,
                                                        nscoord aFontSize,
-                                                       bool aUseUserFontSet);
+                                                       bool aUseUserFontSet,
+                                                       FlushUserFontSet aFlushUserFontSet);
 
   static already_AddRefed<nsFontMetrics> GetMetricsFor(nsPresContext* aPresContext,
                                                        nsStyleContext* aStyleContext,
                                                        const nsStyleFont* aStyleFont,
                                                        nscoord aFontSize,
                                                        bool aUseUserFontSet);
 
   /**