Remove the popover defaultopen attribute

Per the [1] resolution, we have decided to remove this feature
for now, while we work on ways to make it more generic and
applicable to other elements such as `<details>` and `<dialog>`.

[1] openui/open-ui#631 (comment)

Bug: 1307772
Change-Id: I7bc385fcbe1fadfcdf83d2d65c2e46abc07cb8a4

html/semantics/popovers/popover-anchor-display.tentative.html

@@ -3,6 +3,7 @@
 <link rel=author href="mailto:masonf@chromium.org">
 <link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <link rel=match href="popover-anchor-display-ref.tentative.html">
+<script src="resources/popover-utils.js"></script>
 
 <p>There should be a green box attached to the right side of each orange box.</p>
 
@@ -24,6 +25,10 @@
   <div id=popover3 popover=manual defaultopen></div>
 </div>
 
+<script>
+showDefaultopenPopoversOnLoad();
+</script>
+
 <style>
   .ex {
     margin: 25px;

html/semantics/popovers/popover-top-layer-combinations.tentative.html

@@ -29,6 +29,7 @@
 </style>
 
 <script>
+showDefaultopenPopoversOnLoad();
 const isDialog = (ex) => ex instanceof HTMLDialogElement;
 const isFullscreen = (ex) => ex.classList.contains('fullscreen');
 function ensureIsOpenPopover(ex,message) {

html/semantics/popovers/resources/popover-utils.js

@@ -73,3 +73,34 @@ async function blessTopLayer(visibleElement) {
   await wait_click;
   button.remove();
 }
+// This is a "polyfill" of sorts for the `defaultopen` attribute.
+// It can be called before window.load is complete, and it will
+// show defaultopen popovers according to the rules previously part
+// of the popover API: any popover=manual popover can be shown this
+// way, and only the first popover=auto popover.
+function showDefaultopenPopoversOnLoad() {
+  function show() {
+    const popovers = Array.from(document.querySelectorAll('[popover][defaultopen]'));
+    popovers.forEach((p) => {
+        // The showPopover calls below aren't guarded by a check on the popover
+        // open/closed status. If they throw exceptions, this function was
+        // probably called at a bad time.
+        switch (p.popover) {
+        case 'auto':
+          if (!document.querySelector('[popover]:open')) {
+            p.showPopover();
+          }
+          break;
+        case 'manual':
+          p.showPopover();
+        default:
+          assert_notreached(`Unknown popover type ${p.popover}`);
+      }
+    });
+  }
+  if (document.readyState === 'complete') {
+    show();
+  } else {
+    window.addEventListener('load',show,{once:true});
+  }
+}