Svelte slot props not reactive after initial render

Slot Prop Reactivity Issue

In a Svelte 3.x component tree a parent can expose a reactive variable to a child via a slot, e.g. <Child let:val>{val}</Child>. After the first render, updates to val in the parent do not propagate to the slotted content.

Confirmed facts

• The slot prop value is evaluated once during component initialization.
• Reactivity is triggered only when the assignment to the variable is performed (e.g., let:val = $:parentValue or val = newValue).
• If the parent updates parentValue without re‑assigning the slot prop, the child receives the same reference and Svelte does not detect a change.

Likely explanation

The non‑reactivity is expected behavior: slot props are treated like ordinary props. They become reactive only when the parent re‑assigns the prop expression, which causes the compiler to re‑run the slot’s rendering function.

Steps to make slot props reactive

1. Define a reactive variable in the parent, e.g. $:count = 0.
2. Pass it to the child using a slot prop that references the reactive variable: <Child let:count={$:count}>.
3. Inside the child, use the slot prop directly; any change to count will cause the slot to re‑render because the prop expression is re‑evaluated.
4. If the value is an object, mutate it immutably (assign a new object) or wrap it in a store so that a top‑level assignment occurs.

Verification: create a child that updates a variable with setInterval and pass it via let:updating={$:updating}. The DOM should reflect each change without a full parent re‑render.

Missing diagnostic

Please confirm whether the parent updates the variable using a reactive assignment ($:) or by directly assigning a new value to the slot prop.