Move hide event before state changes on :open

Move the timing of the popover `popoverhide` event a bit earlier
in the process, just *before* the state changes to "transitioning".
This has the effect of making the state clean (either :open or
:closed) for both `popoverhide` and `popovershow`.

See the discussion here:
openui/open-ui#607

Bug: 1307772
Change-Id: I5aaabfd48204e54831ce435c71a54e692e139caf

html/semantics/popovers/popover-events.tentative.html

@@ -30,8 +30,14 @@
           assert_false(popover.hasAttribute('onpopoverhide'));
           t.add_cleanup(() => popover.removeAttribute('onpopovershow'));
           t.add_cleanup(() => popover.removeAttribute('onpopoverhide'));
-          popover.onpopovershow = () => ++showCount;
-          popover.onpopoverhide = () => ++hideCount;
+          popover.onpopovershow = (p) => {
+            assert_true(p.matches(':closed'),'The popover should be in the :closed state when the popovershow event fires.');
+            ++showCount;
+          };
+          popover.onpopoverhide = () => {
+            assert_true(p.matches(':open'),'The popover should be in the :open state when the popoverhide event fires.');
+            ++hideCount;
+          };
           break;
         default: assert_unreached();
       }