Bug 1796408 - Restore previous timing. r=Gijs

Big hack, but this ensures that the dialog is at the right position when it's a
subdialog positioned by the front-end... Otherwise we fire this before setting
the vertical margin in the opener, and sadness ensues.

I'd try to find a nicer solution to this, but:

 * This is the easiest that is potentially upliftable.
 * I'm a bit swamped with some other work.
 * This is an extra one-liner, for a very niche feature.

Differential Revision: https://phabricator.services.mozilla.com/D160420

toolkit/content/widgets/dialog.js

@@ -343,6 +343,11 @@
         return;
       }
       try {
+        // FIXME(emilio, bug 1797624): This setTimeout() ensures enough time
+        // has passed so that the dialog vertical margin has been set by the
+        // front-end. For subdialogs, cursor positioning should probably be
+        // done by the opener instead, once the dialog is positioned.
+        await new Promise(r => setTimeout(r, 0));
         await window.promiseDocumentFlushed(() => {});
         window.notifyDefaultButtonLoaded(defaultButton);
       } catch (e) {}