A right click on a diff line opens the place it names: its file, the hunk's new line, the line itself

In a diff pane (a .diff/.patch file, or a command pane's diff output), the
look's own expansion, which its hover shows, takes a whole line for a
`diff --git`, `---`, `+++` or `@@` line wherever the pointer is on it,
markers included, and for a hunk line when the pointer is on its `+`, `-`
or space. Then the look opens the file (a/ b/ dropped, timestamps cut) at
the line from the hunk header and the lines above it: `@@` the hunk's
first new line, context and added lines their own, a removed line the new
line now where it was. `diff --git` and `+++` open the new file; `---` the
old one, unless the `+++` under it names another. On a hunk line's code
the expansion is its words, the prefix never one of them, and looks as
ever. Other panes are untouched.

Paths resolve in the repository (walking up to `.git` or `.jj` from the
diff's directory, or a command pane's), then that directory. A file not
here opens nothing and says `Look: b/x: no such file here`, or `deleted by
this diff`. A 9P look writing a whole line of the diff does the same.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

2 files changed, 41 insertions, 2 deletions

diff --git a/test/snapshots/diffview.golden b/test/snapshots/diffview.golden
index 2da9f6e3..9e171fd8 100644
--- a/test/snapshots/diffview.golden
+++ b/test/snapshots/diffview.golden
@@ -54,6 +54,21 @@
 |28: 0-0 d,#608269, 1-99 d,#0d1410,
 |29: 0-0 d,#608269, 1-99 d,#0d1410,
 |30: 0-0 d,#608269, 1-99 d,#0d1410,
+== snap look-added-line grid=100x31 cursor=55,22
+|16:    /tmp/pardes-snap/diffview/cwd/src/shape.zig Save Tty Collapse Del TreeContext
+|17:      1 const std = @import("std");
+|18:      2
+|19:      3 pub const Shape = union(enum) {
+|20:      4     circle: f32,
+|21:      5     square: f32,
+|22:      6     triangle: struct { base: f32, height: f32 },
+|23:      7
+|24:      8     const note =
+|25:      9         \\areas, defer
+|26:     10         \\summed
+|27:     11     ;
+|28:     12 };
+|29:     13
 == snap command-pane grid=100x31 cursor=7,3
 |Newcol Joincol Find Grep Help Changelog Tutor Dump Themes Config Debug Exit
 |   New Tty Find Grep Joincol Delcol
@@ -117,3 +132,17 @@
 |28: 0-0 d,#608269, 1-2 d,#0d1410, 3-99 #e5eee5,#0d1410,
 |29: 0-0 d,#608269, 1-1 d,#0d1410, 2-99 #e5eee5,#0d1410,
 |30: 0-0 d,#152019, 1-99 d,#0d1410,
+== snap command-look grid=100x31 cursor=23,10
+|3:      1 cat change.diff
+|5:    /tmp/pardes-snap/diffview/cwd/src/shape.zig Save Tty Collapse Del TreeContext
+|6:      1 const std = @import("std");
+|7:      2
+|8:      3 pub const Shape = union(enum) {
+|9:      4     circle: f32,
+|10:      5     square: f32,
+|11:      6     triangle: struct { base: f32, height: f32 },
+|12:      7
+|13:      8     const note =
+|14:      9         \\areas, defer
+|15:     10         \\summed
+|16:     11     ;
diff --git a/test/snapshots/diffview.snap b/test/snapshots/diffview.snap
index b5900db9..23d060b7 100644
--- a/test/snapshots/diffview.snap
+++ b/test/snapshots/diffview.snap
@@ -1,7 +1,9 @@
 # A diff pane, for reviewing a change: each hunk's code coloured as Zig (a
 # multi-line string over three lines, `defer` inside it no keyword), added and
 # removed rows tinted to the pane's edge with their prefix in the tint's hue,
-# headers muted.
+# headers muted. Then a right click on an added line's `+` opens the new file
+# at that line: the path is the repository's (a `.git` above the diff), b/
+# dropped.
 dirmk .git
 dirmk src
 file src/shape.zig const std = @import("std");\n\npub const Shape = union(enum) {\n    circle: f32,\n    square: f32,\n    triangle: struct { base: f32, height: f32 },\n\n    const note =\n        \\\\areas, defer\n        \\\\summed\n    ;\n};\n
@@ -11,9 +13,13 @@ wait 8000 Newcol Joincol
 stable 700 20000
 snap diff-pane
 snapstyle diff-colours
+press right 8 15
+release right 8 15
+stable 700 15000
+snap look-added-line
 # The same diff printed by a command (`git diff`, here `cat`) into a command
 # pane: its output is told for a diff and coloured the same, over the colours
-# the program printed.
+# the program printed, and a right click on a `+` there opens the file too.
 file cmds.txt cat change.diff\n
 start 31 100 cmds.txt
 wait 8000 cmds.txt
@@ -25,3 +31,7 @@ wait 10000 exit 0
 stable 700 15000
 snap command-pane
 snapstyle command-colours
+press right 3 21
+release right 3 21
+stable 700 15000
+snap command-look