Start an idle period more often

In particular, an idle period should start in the case where there are
tasks, but they are all associated with non-fully-active documents.
Fixes #4212.

Author: rmcilroy

source

@@ -89490,7 +89490,8 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
    <li><p>If this is a <span>browsing context</span> <span>event loop</span> (as opposed to a <a
    href="#workers">worker</a> <span>event loop</span>), and there are no <span
    data-x="concept-task">tasks</span> in the <span>event loop</span>'s <span data-x="task
-   queue">task queues</span>, and the <span>event loop</span>'s <span>microtask queue</span> is
+   queue">task queues</span> which are associated with a <code>Document</code> that is
+   <span>fully active</span>, and the <span>event loop</span>'s <span>microtask queue</span> is
    empty, and none of the <span data-x="browsing context">browsing contexts</span> have a
    <span>rendering opportunity</span>, then for each <span>browsing context</span>, run the steps in
    the <span>start an idle period algorithm</span>, passing the <code>Window</code> associated with