# Objective

Fixes #58104.

When using the `fold_at_level` action (default keybind: Ctrl-K
Ctrl-(1-9) on Windows) on a function with its arguments split across
multiple lines (see the example in #58104), only the arguments would be
folded, while the function body remained expanded and only blocks at the
next level inside the function were folded.

To reproduce:

1. Open a `.rs` file (or any language with similar folding behavior).
2. Add a function with multiline arguments:

```rust
fn test(
    a: i32,
) {
    let b = a;
}
```

3. Run `fold_at_level` with level `1`.
4. Observe that the argument list is folded, but the function body
remains expanded.

## Solution

When iterating through foldable ranges, `fold_at_level` advanced the
search position to the line after the folded crease. This caused it to
skip any crease starting on the same line as the end of the previous
one. The fix keeps the search position on the last line of the folded
crease instead, allowing consecutive creases to be processed.

## Testing

Automated: Added 1 test covering chained foldable blocks, including
functions with multiline arguments and if/else blocks.

Manual: Verified that invoking `fold_at_level(1)` on a function with
multiline arguments folds both the argument list and the function body.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

**Before:**

<img width="211" height="107" alt="image"
src="https://github.com/user-attachments/assets/dbfbbdfa-faee-42f3-ac25-0581a796e124"
/>

**After:**

<img width="221" height="41" alt="image"
src="https://github.com/user-attachments/assets/6746b341-0587-48a4-908a-e743c974cae9"
/>

## Release Notes:

- Fixed: `fold_at_level` not folding function bodies when arguments span
multiple lines

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>