Native select field with multiple uses duplicate query param

[johnsicili]

When submitting a form with multiple options selected in a select list, each selection gets a query param key value, but each key is the same. When accessing the value in the system.search var, only the last value shows up.

I think a solution is to comma separate the values and use one key.

Repro:

1. Add form
2. Add select
3. Enable multiple
4. Submit form
5. Inspect system.search var and see only one value.

https://github.com/user-attachments/assets/169fed2b-d19c-465c-8601-563dfce0740d

[kof]

Query string is correct, but seems like we are not using URLSearchParams parser, which correctly parses search string, if you put into the browser ?a=1&a=2

const search = new URLSearchParams(location.search)
for (const value of search) {
  console.log(value)
}

Will log:
(2) ['a', '1']
(2) ['a', '2']

[kof]

Turns out we are using a flat object because in expressions the autocompletion wouldn't work otherwise.