# Objective

Fixes #60192
Closes https://github.com/zed-industries/zed/issues/62308

`editor: align selection` lines cursors up by their buffer column, and
that column counts bytes. If a multi-byte character sits before the
cursor, the byte column is larger than the position the cursor is
actually drawn at, so the row gets padded with the wrong number of
spaces.

The issue reports it with `←` (3 bytes) and `π` (2 bytes):

```
a ← 1  # one
bc ← π  # two
```

Put a cursor on each `#`, run the action, and the result is still
misaligned:

```
a ← 1    # one
bc ← π  # two
```

This is not the columnar selection bug fixed in #57097. That one was
`select_columns` in `selection.rs`, where the output is a selection
range. This one is `align_selections` in `editor.rs`, where the output
is inserted spaces, so the same byte-column assumption was left behind
in a second place, and fixing it here needs a rounding step that the
first fix did not.

## Solution

Measure each cursor by its x offset in the laid-out display row
(`DisplaySnapshot::x_for_display_point`), take the target for a column
as the furthest x across the rows, then turn the difference into whole
spaces by dividing by the advance width of `' '`. The offset that
carries into later columns becomes an x offset instead of a column
count.

The display map has already expanded tabs by the time the row is laid
out, so a leading tab now counts as its expanded width instead of as a
single byte.

Two things I would look at first in review:

- The division rounds instead of truncating. The x offsets are built by
repeated float addition, so a gap that should be exactly three spaces
can arrive as 2.9999998, and truncating inserts two.
- The function returns early if the space advance is missing or zero.
Dividing by zero gives `inf`, which saturates to a huge `u32` and then
tries to allocate that many spaces.

I did not add any public items and did not touch `selection.rs`.

## Testing

`cargo test -p editor align` on Windows: 6 passed, 0 failed. That is the
new test plus the two existing `align_selections` tests, which I did not
change and which still pass.

`test_align_selections_with_multibyte_chars` covers the repro from the
issue, a second column whose offset has to carry past a multi-byte
character in the first, a leading tab, a non-BMP character, and a case
where multi-byte characters sit after the cursors and nothing should
move.

I also checked that the test catches the bug rather than just passing:
reverting the change in `editor.rs` and keeping the test makes it fail
on the repro, inserting four spaces where three are right. Putting the
change back makes it pass. The two older align tests pass either way,
since they are pure ASCII.

What I have not covered:

- Wide CJK characters, combining marks, and ZWJ clusters. These should
be right by construction, since the code measures advances rather than
counting characters, but I have no tests for them. The headless text
system behind `gpui::test` gives every BMP character the same advance,
so a test there would assert the test double's behavior rather than the
real renderer's.
- Proportional fonts. Aligning with inserted spaces cannot be exact when
glyph widths vary. The code rounds to the nearest whole space.
- Soft-wrapped rows. I measure x from the start of the wrapped row but
still group cursors by buffer row, so two cursors on one buffer row that
sit either side of a wrap boundary get measured from different origins,
and the carried offset crosses that boundary as if they shared one. The
old byte-column code did not have that particular failure. I left it
alone because fixing it is a different change, but I would rather flag
it than have you find it.
- I work on Windows and have no macOS machine. The arithmetic is
platform independent, so I do not expect a difference, but I have not
checked.

To try it: paste the two lines from the issue, put a cursor on each `#`
with `editor: select next`, then run `editor: align selection`. The two
`#` should line up.

## Self-Review Checklist:

- [ ] 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 `editor: align selections` misaligning rows and Vim `ctrl-d` /
`ctrl-u` / `ctrl-f` leaving the cursor behind on lines with multi-byte
characters or tabs.

---------

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