1348906 - Stop using observer notifications for before-first-paint notifications to tabchild and zoomconstraintsclient consumers

Status: NEW

(Core :: Layout, enhancement, P3)

Description

[:Gijs (he/him)]

TabChildren and ZoomConstraintsClients listen for before-first-paint observer notifications.

Unfortunately, *every* tabchild does this, and every single one creates a listener that won't go away until its RecvDestroy() method is called. The actual Observe() implementation looks like this:

https://dxr.mozilla.org/mozilla-central/rev/1b9293be51637f841275541d8991314ca56561a5/dom/ipc/TabChild.cpp#456,461


>  if (!strcmp(aTopic, BEFORE_FIRST_PAINT)) {
    if (AsyncPanZoomEnabled()) {
      nsCOMPtr<nsIDocument> subject(do_QueryInterface(aSubject));
      nsCOMPtr<nsIDocument> doc(GetDocument());

>      if (SameCOMIdentity(subject, doc)) {
        nsCOMPtr<nsIPresShell> shell(doc->GetShell());
        if (shell) {
          shell->SetIsFirstPaint(true);
        }

        APZCCallbackHelper::InitializeRootDisplayport(shell);
      }
    }
  }

This looks very much like it should be using some other pattern to have the presshell notify the TabChild. Right now every tabchild in the process will get these notifications, and every single one will have to check whether the document matches. That's O(n) for something that should be O(1).

The ZoomConstraintsClient has exactly the same problem.

https://dxr.mozilla.org/mozilla-central/rev/1b9293be51637f841275541d8991314ca56561a5/layout/base/ZoomConstraintsClient.cpp#151,153