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In some circumstances, after hitting New Identity the new browser window will open at a different location on the screen before the old one is gone. We are currently doing:

  OpenBrowserWindow();

  torbutton_log(3, "New identity successful");

  // Run garbage collection and cycle collection after window is gone.
  // This ensures that blob URIs are forgotten.
  window.addEventListener("unload", function (event) {
    torbutton_log(3, "Initiating New Identity GC pass");
    // Clear out potential pending sInterSliceGCTimer:
    m_tb_domWindowUtils.runNextCollectorTimer();

    // Clear out potential pending sICCTimer:
    m_tb_domWindowUtils.runNextCollectorTimer();

    // Schedule a garbage collection in 4000-1000ms...
    m_tb_domWindowUtils.garbageCollect();

    // To ensure the GC runs immediately instead of 4-10s from now, we need
    // to poke it at least 11 times.
    // We need 5 pokes for GC, 1 poke for the interSliceGC, and 5 pokes for CC.
    // See nsJSContext::RunNextCollectorTimer() in
    // https://mxr.mozilla.org/mozilla-central/source/dom/base/nsJSEnvironment.cpp#1970.
    // XXX: We might want to make our own method for immediate full GC...
    for (let poke = 0; poke < 11; poke++) {
       m_tb_domWindowUtils.runNextCollectorTimer();
    }

    // And now, since the GC probably actually ran *after* the CC last time,
    // run the whole thing again.
    m_tb_domWindowUtils.garbageCollect();
    for (let poke = 0; poke < 11; poke++) {
       m_tb_domWindowUtils.runNextCollectorTimer();
    }

    torbutton_log(3, "Completed New Identity GC pass");
  });

  // Close the current window for added safety
  window.close();

So, this issue can show up if the garbage collection takes quite some time for some reason. We might want to think about whether we can improve things for this use case while not making New Identity much slower.