[BUG] `position-try-fallbacks` algorithm triggered more eagerly than spec

[jamesnw]

Describe the bug
In the spec and implementations, the position-try-fallbacks algorithm is only run when the currently applied fallback overflows. This differs from the polyfill, pre-release versions of Safari, and Floating UI, which falls back to the initial position as soon as possible.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://anchor-polyfill.netlify.app/#position-try
2. Scroll to apply a fallback
3. Polyfill: Scroll back slightly, and the target jumps back to the initial position immediately.
4. Spec, implementations: Scroll back slightly, and the target does not return to the initial position.

Expected behavior
w3c/csswg-drafts#12682