Objective

Fixes #60192
Closes #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
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• 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.