Reactive declarations not reflecting binding values before onMount

[Rich-Harris]

Describe the bug

Reactive declarations that depend on values that are initialised from bindings are incorrect during onMount.

To Reproduce

https://svelte.dev/repl/1d7a000815544ddbb53044b949596ed6?version=3.9.1

Expected behavior

I expected w and h to be correct by the time the onMount code ran. I'm not certain if that's a reasonable expectation.

Severity

Meh

[ghost]

Also doesn't work with afterUpdate, which surprised me.
https://svelte.dev/repl/b55383ad34ea445cb36121118a8843c8?version=3.9.1

[Conduitry]

This seems related to #2816, but I'm not positive it's exactly the same, as that issue involved component prop binding. Like that issue, this one can also be worked around by waiting a tick at the beginning of onMount.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been closed as it was previously marked as stale and saw no subsequent activity.