Bug 1439954 - Make gfxPlatform::ForceGlobalReflow when called in a content process trigger reflows only within that process; it should not try (and fail, with an assertion) to affect the parent or other content processes. r=jrmuizel

gfx/thebes/gfxPlatform.cpp

@@ -2177,10 +2177,21 @@ gfxPlatform::FlushFontAndWordCaches()
 /* static */ void
 gfxPlatform::ForceGlobalReflow()
 {
-    // modify a preference that will trigger reflow everywhere
+    MOZ_ASSERT(NS_IsMainThread());
+    if (XRE_IsParentProcess()) {
+        // Modify a preference that will trigger reflow everywhere (in all
+        // content processes, as well as the parent).
         static const char kPrefName[] = "font.internaluseonly.changed";
         bool fontInternalChange = Preferences::GetBool(kPrefName, false);
         Preferences::SetBool(kPrefName, !fontInternalChange);
+    } else {
+        // Send a notification that will be observed by PresShells in this
+        // process only.
+        nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
+        if (obs) {
+            obs->NotifyObservers(nullptr, "font-info-updated", nullptr);
+        }
+    }
 }
 
 void

layout/base/PresShell.cpp

@@ -1023,6 +1023,7 @@ PresShell::Init(nsIDocument* aDocument,
       if (XRE_IsParentProcess() && !sProcessInteractable) {
         os->AddObserver(this, "sessionstore-one-or-no-tab-restored", false);
       }
+      os->AddObserver(this, "font-info-updated", false);
     }
   }
 
@@ -1260,6 +1261,7 @@ PresShell::Destroy()
       if (XRE_IsParentProcess()) {
         os->RemoveObserver(this, "sessionstore-one-or-no-tab-restored");
       }
+      os->RemoveObserver(this, "font-info-updated");
     }
   }
 
@@ -9326,6 +9328,11 @@ PresShell::Observe(nsISupports* aSubject,
     return NS_OK;
   }
 
+  if (!nsCRT::strcmp(aTopic, "font-info-updated")) {
+    mPresContext->ForceReflowForFontInfoUpdate();
+    return NS_OK;
+  }
+
   NS_WARNING("unrecognized topic in PresShell::Observe");
   return NS_ERROR_FAILURE;
 }

layout/base/nsPresContext.cpp

@@ -233,6 +233,15 @@ nsPresContext::PrefChangedUpdateTimerCallback(nsITimer *aTimer, void *aClosure)
     presContext->UpdateAfterPreferencesChanged();
 }
 
+void
+nsPresContext::ForceReflowForFontInfoUpdate()
+{
+  // We can trigger reflow by pretending a font.* preference has changed;
+  // this is the same mechanism as gfxPlatform::ForceGlobalReflow() uses
+  // if new fonts are installed during the session, for example.
+  PreferenceChanged("font.internaluseonly.changed");
+}
+
 static bool
 IsVisualCharset(NotNull<const Encoding*> aCharset)
 {

layout/base/nsPresContext.h

@@ -1250,6 +1250,10 @@ protected:
   static bool NotifyDidPaintSubdocumentCallback(nsIDocument* aDocument, void* aData);
 
 public:
+  // Used by the PresShell to force a reflow when some aspect of font info
+  // has been updated, potentially affecting font selection and layout.
+  void ForceReflowForFontInfoUpdate();
+
   void DoChangeCharSet(NotNull<const Encoding*> aCharSet);
 
   /**