Bug 1578154 - Ensure all device contexts flush their nsFontCache when the platform font list is reinitialized. r=jwatt

Differential Revision: https://phabricator.services.mozilla.com/D45432

--HG--
extra : moz-landing-system : lando

gfx/thebes/gfxPlatformFontList.cpp

@@ -400,6 +400,13 @@ nsresult gfxPlatformFontList::InitFontList() {
 
   gfxPlatform::PurgeSkiaFontCache();
 
+  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+  if (obs) {
+    // Notify any current presContexts that fonts are being updated, so existing
+    // caches will no longer be valid.
+    obs->NotifyObservers(nullptr, "font-info-updated", nullptr);
+  }
+
   mAliasTable.Clear();
   mLocalNameTable.Clear();
 

layout/base/nsPresContext.cpp

@@ -131,6 +131,10 @@ bool nsPresContext::IsDOMPaintEventPending() {
 }
 
 void nsPresContext::ForceReflowForFontInfoUpdate() {
+  // Flush the device context's font cache, so that we won't risk getting
+  // stale nsFontMetrics objects from it.
+  DeviceContext()->FlushFontCache();
+
   // 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.