# Objective

Closes #62617

Turns out #62617 is just a special trigger point of a more general
issue: replacing a line's contents can silently rewrite the line's
indentation with the auto-indent suggestion.

Consider the following Rust code, where the line has an extra tab,
making its indent 8 spaces instead of the default 4:
```Rust
fn main() {
        println!("hello world");
}
```
If we select and replace the line's contents (without the indentation):
```Rust
fn main() {
        «println!("hello world");»
}
```
with `let a = 8;`, the result is:
```Rust
fn main() {
    let a = 8;
}
```
The extra indent has been stripped.

Tracing this down to `Buffer::edit_internal()` in
`crates/language/src/buffer.rs`, the code decides whether the edited
line needs an indent update via the `first_line_is_new` flag, which ends
up as the `old_row` of an `AutoindentRequestEntry`. One of these checks
is:


https://github.com/zed-industries/zed/blob/cdc537c690e605b4a061b2e99c3d292a1d4b7145/crates/language/src/buffer.rs#L2913-L2918

When replacing a line's contents, the edit range ends exactly at the end
of the line, so `old_start.column + (range_len as u32) == old_line_end`.
Because the check uses `<`, this case meets none of the these
conditions, `first_line_is_new` stays `true`, and an indent update is
triggered. If the manual indent differs from the suggested indent, it
gets overwritten — exactly as in the example above.

For IME input, composition updates replace the previously marked preedit
text, which sits at the end of the line — the same geometry as a full
line-content replacement. In some environments (observed on KDE Wayland
with fcitx), a single keystroke delivers the preedit update twice, so
the replacement happens on the very first keystroke, which is what
#62617 reports. On other platforms, the replacement may happens once the
composition changes, i.e. on the second keystroke, so it takes at least
two characters to trigger.

## Solution

Simply change the guard from `(old_start.column + (range_len as u32) <
old_line_end` to `(old_start.column + (range_len as u32) <=
old_line_end`.

## Testing

Two new tests are added: `test_ime_composition_keeps_manual_indent`
covers the IME input path, and
`test_replacing_line_content_keeps_manual_indent` covers a plain
line-content replacement.

## 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

---

Release Notes:

- Fixed manual indentation being lost when replacing a line's contents
or typing with an input method