The removal of centred alignment mode introduced a new flag called is_modal. This flag is only supposed to be unset in close, so that removing a modal dialog from the document and then reinsert it keeps its modal status, even without calling ShowModal() again explicitly.

Here's the test for it https://searchfox.org/mozilla-central/source/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/abspos-dialog-layout.html#146-159.

The is_modal (https://html.spec.whatwg.org/multipage/#is-modal) flag
allows dialog to keep its modal status after it's being removed
from the document, so that it can be added back as a modal dialog
without calling ShowModal() explicitly.

In Gecko, NS_EVENT_STATE_MODAL_DIALOG state is the flag.

There's a r+ patch which didn't land and no activity in this bug for 2 weeks.
:sefeng, could you have a look please?
For more information, please visit auto_nag documentation.

We are planning to with https://github.com/whatwg/html/issues/6216, so this bug and the patch are no longer needed.