fix(datetime): fix content sometimes not loading due to race condition

[averyjohnston]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: Resolves #24112

When datetime is used in a modal etc, there is a race condition between the datetime instance loading and the modal animation starting. This sometimes causes the intersectionObservers that handle the datetime-ready class to erroneously fire with a visibility of 0 from the beginning, so datetime-ready is never added.

What is the new behavior?

The observers that flag whether the datetime instance is ready now wait a frame before starting.

Does this introduce a breaking change?

• Yes
• No

Other information

[liamdebeasi]

Great job!

[sean-perkins]

Looks good! 👏