First pass at a very dumb two file diff viewer

[jcmuller]

A vimdiff-style side-by-side diff viewer.

Usage

• :diff-open file1 file2 opens both files in a vertical split and starts a diff session. Alias: :diffs.
• hx --diff file1 file2 (or -d) does the same from the command line.
• :diff-this marks a view; call it in a second view to pair them.

Inside a session

• :diffget / reset-diff-change pulls the hunk at the cursor from the partner file.
• :diffput pushes the current selection's hunk the other way.
• :diff-off tears the session down, leaving both views open.
• ]g and [g jump between hunks, using session hunks and falling back to VCS hunks when outside a session.

How it works

The renderer recomputes hunks when Document::version() changes. A LineAnnotation inserts virtual filler lines so both sides line up by logical row. Line backgrounds come from diff.plus, diff.minus, and diff.delta. Intra-line highlights use an OverlayHighlights layer keyed on diff.delta.text, falling back to diff.delta when unset. Changed regions are diffed at the word level (alphanumeric runs), so foo_bar vs baz_bar highlights foo rather than a run of character edits.

Theme

diff.delta.text is a new scope for intra-line changed characters; it falls back to diff.delta when unset. The default theme.toml defines it with an explicit bg so whitespace changes are visible against the diff.delta line background.

Tests

23 unit tests in helix-view/src/diff_session.rs cover alignment, hunk intersection, and the diffget/diffput transaction builders. Integration tests in helix-term/tests/test/diff_mode.rs exercise :diff-open, :diff-this, :diff-off, :diffget, and :diffput against tempfile-backed documents.

Known rough edges

• Clicking on a filler line places the cursor above the intended row; click coordinates need remapping for virtual lines.
• Intra-line highlights won't show up well if the active theme has low contrast between diff.delta's fg and bg.
• A pure insertion at line 0 renders filler after the first line rather than before it.
• Horizontal scroll, tabs, and wide characters aren't accounted for in filler column math.

[archseer]

Great work, this is awesome to see! I'll test drive the branch over the week

[archseer]

Here's the default theme config I'd like to see:

"diff.plus" = { bg = "#1f6f4e" } # shifted down by 20% on hsl
"diff.minus" = { bg = "#ad0b55" } # shifted down by 20% on hsl
"diff.delta" = { bg = "#41288c" }
"diff.delta.text" = { bg = "#5e3bc6" }
"diff.delta.conflict" = { fg = "#f22c86", bg = "#6a2038" }

"diff.plus.gutter" = { fg = "#35bf86" }
"diff.minus.gutter" = { fg = "#f22c86" }
"diff.delta.gutter" = { fg = "#6f44f0" }

I haven't adjusted diff.delta.conflict since I haven't seen it in practice yet.

Bugs so far:

• ]g seems to get stuck if your cursor is already on a changed area. Scrolling past it will allow you to correctly jump to the next change.
• There seems to be some issues with diff alignment after removed lines. My test case is hx -d hosts/cerulean/kernel.config hosts/trantor/kernel.config under https://github.com/archseer/snowflake, it seems to align the first two deletions in the file correctly, but then the further changed blocks are slightly offset:

[archseer]

Also, only scroll position is updated but it would be good if we kept the cursor line position in sync as well. If I scroll halfway through a file then space-w-w to the other file I'd expect the cursor to stay on the same relative line position (e.g. if I am on line 4 and the file on the left lines up so that the same visual line is line 2 in the other file, I'd expect to go to line 2). That way the view stays fixed as I switch between the files. Currently, cursor will jump to the last known position, which often ends up scrolling both views and can be disorientating (e.g. if the other file was untouched, both views will jump to the start of the file).

[m4rch3n1ng]

would it be possible to have more dedicated theme keys for this? something like diff.delta.line, diff.plus.line and diff.delta.text or even more explicit diff.delta.view.line, ..., or something like that? 'cause i don't really want to change the diff.delta/diff.plus/etc highlights, as that also changes how the space+g menu and the diff language is rendered.

[m4rch3n1ng]

thinking it over, i think my preference would be:

Scope	Purpose
diff.delta.view	Modified line background
diff.delta.view.text	Intra-line changed characters
diff.minus.view	Deleted line background
diff.plus.view	Added line background

doing it like that and applying the styles directly also makes it, so i can theoretically highlight the diffs in the diff view via text color instead of being forced to use them as background color