Bug 1588791 - Make fission iframes honor and deal with the scrolling attribute. r=mattwoodrow

Differential Revision: https://phabricator.services.mozilla.com/D59630

dom/html/nsGenericHTMLFrameElement.cpp

@@ -8,6 +8,7 @@
 
 #include "mozilla/dom/HTMLIFrameElement.h"
 #include "mozilla/dom/XULFrameElement.h"
+#include "mozilla/dom/BrowserBridgeChild.h"
 #include "mozilla/dom/WindowProxyHolder.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/PresShell.h"
@@ -291,10 +292,15 @@ nsresult nsGenericHTMLFrameElement::AfterSetAttr(
   if (aNameSpaceID == kNameSpaceID_None) {
     if (aName == nsGkAtoms::scrolling) {
       if (mFrameLoader) {
-        // FIXME(bug 1588791): This should work for fission iframes.
+        ScrollbarPreference pref = MapScrollingAttribute(aValue);
         if (nsIDocShell* docshell = mFrameLoader->GetExistingDocShell()) {
-          nsDocShell::Cast(docshell)->SetScrollbarPreference(
-              MapScrollingAttribute(aValue));
+          nsDocShell::Cast(docshell)->SetScrollbarPreference(pref);
+        } else if (auto* child = mFrameLoader->GetBrowserBridgeChild()) {
+          // NOTE(emilio): We intentionally don't deal with the
+          // GetBrowserParent() case, and only deal with the fission iframe
+          // case. There's no reason not to make it work, but it's a bit of
+          // boilerplate for something that we don't use...
+          child->SendScrollbarPreferenceChanged(pref);
         }
       }
     } else if (aName == nsGkAtoms::mozbrowser) {