fix: keep default values of props a proxy after reassignment

[paoloricciuti]

Svelte 5 rewrite

Closes #11859

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[Rich-Harris]

This might seem like a side-issue but for me this is the straw that broke the camel's back: serialize_set_binding is completely out of control and I think we need to totally refactor it. I'm looking at the code and it's just so hard to follow — it's not the worst part of our codebase but it's up there. Does anyone else share this concern?

As a more immediate matter, initial_bindable_proxy doesn't really accurately tell me what kind of value this is and in what circumstances it would be true or false — it's unrelated to whether it's even a prop, let alone whether it's bindable. I think it should have a more descriptive name, and we should try to avoid things like binding.initial && (as opposed to binding.initial !== null &&)

[paoloricciuti]

This might seem like a side-issue but for me this is the straw that broke the camel's back: serialize_set_binding is completely out of control and I think we need to totally refactor it. I'm looking at the code and it's just so hard to follow — it's not the worst part of our codebase but it's up there. Does anyone else share this concern?

Working on it for this PR didn't seem completely crazy but there's definitely a lot in there.

As a more immediate matter, initial_bindable_proxy doesn't really accurately tell me what kind of value this is and in what circumstances it would be true or false — it's unrelated to whether it's even a prop, let alone whether it's bindable. I think it should have a more descriptive name, and we should try to avoid things like binding.initial && (as opposed to binding.initial !== null &&)

You are right: I could think of a better name and initially I thought this was just for bindable properties. I think a better name could be is_initial_proxied maybe? And obviously I can refactor to the more precise bind.initial !==null and keep a not for future PRs.

However at this point what do we want to do with the method? Should I refactor the name and check and then we think how we want to handle the method either in this or another PR?

[Rich-Harris]

I guess a more comprehensive refactor could take some time, and in the meantime there's a bug. So yeah, let's go ahead with those changes — thanks!