[popover] Handle showPopover changing popover attribute during before…

…toggle

https://bugs.webkit.org/show_bug.cgi?id=254268

Reviewed by Tim Nguyen.

Handle showPopover changing popover attribute during beforetoggle by throwing an exception:
whatwg/html#9058

* LayoutTests/imported/w3c/web-platform-tests/html/semantics/popovers/popover-attribute-basic-expected.txt:
* LayoutTests/platform/ios-wk2/imported/w3c/web-platform-tests/html/semantics/popovers/popover-attribute-basic-expected.txt:
* Source/WebCore/html/HTMLElement.cpp:
(WebCore::HTMLElement::showPopover):

Canonical link: https://commits.webkit.org/262026@main

LayoutTests/imported/w3c/web-platform-tests/html/semantics/popovers/popover-attribute-basic-expected.txt

@@ -25,7 +25,7 @@ PASS Removing a visible popover=auto element from the document should close the
 PASS A showing popover=auto does not match :modal
 PASS Removing a visible popover=manual element from the document should close the popover
 PASS A showing popover=manual does not match :modal
-FAIL Changing the popover type in a "beforetoggle" event handler should throw an exception (during showPopover()) assert_throws_dom: function "() => popover.showPopover()" did not throw
+PASS Changing the popover type in a "beforetoggle" event handler should throw an exception (during showPopover())
 PASS Changing the popover type in a "beforetoggle" event handler should throw an exception (during hidePopover())
 PASS Changing a popover from auto to auto (via attr), and then auto during 'beforetoggle' works
 PASS Changing a popover from auto to auto (via attr), and then manual during 'beforetoggle' works

LayoutTests/platform/ios-wk2/imported/w3c/web-platform-tests/html/semantics/popovers/popover-attribute-basic-expected.txt

@@ -25,7 +25,7 @@ PASS Removing a visible popover=auto element from the document should close the
 PASS A showing popover=auto does not match :modal
 PASS Removing a visible popover=manual element from the document should close the popover
 PASS A showing popover=manual does not match :modal
-FAIL Changing the popover type in a "beforetoggle" event handler should throw an exception (during showPopover()) assert_throws_dom: function "() => popover.showPopover()" did not throw
+PASS Changing the popover type in a "beforetoggle" event handler should throw an exception (during showPopover())
 PASS Changing the popover type in a "beforetoggle" event handler should throw an exception (during hidePopover())
 PASS Changing a popover from auto to auto (via attr), and then auto during 'beforetoggle' works
 PASS Changing a popover from auto to auto (via attr), and then manual during 'beforetoggle' works

Source/WebCore/html/HTMLElement.cpp

@@ -1344,7 +1344,7 @@ ExceptionOr<void> HTMLElement::showPopover()
         document().hideAllPopoversUntil(ancestor.get(), FocusPreviousElement::No, FireEvents::Yes);
 
         if (popoverState() != originalState)
-            return { };
+            return Exception { InvalidStateError, "The value of the popover attribute was changed while hiding the popover."_s };
 
         if (auto check = checkPopoverValidity(*this, PopoverVisibilityState::Hidden); check.hasException())
             return check.releaseException();