fix: overwrite #last_value when Spring is .set() with {instant: true}

[grieben]

Fixes #14645

After using the {instant: true} option with Spring.set(), a subsequent (non-instant) .set() sometimes displayed unexpected behavior. See Issue link for reproduction, .gif of example, and more details.

This PR overwrites the Spring's #last_value after updating instantly so that no data/context from before the instant .set() lingers to interfere with future .set()s. After building this modified version of svelte and linking it to a sveltekit project, I no longer see the glitchy behavior described in the Issue linked above.

I saw no tests in the /svelte/tests/ directory that use the newly created Spring class and don't feel currently knowledgeable enough to write a full suite myself. I'm also unsure how I would test the private #last_value property. If tests are necessary, please let me know and I will do my best to provide them.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 09ab0e9

The changes in this PR will be included in the next version bump.

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svelte	Patch

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[Rich-Harris]

thank you!