Expand the meld-3 tool experiment to editing file conflicts

CHANGELOG.md

@@ -64,6 +64,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * Added completions for [Nushell](https://nushell.sh) to `jj util completion`
 
+* Another way to resolve conflicts with Meld, in 3 tabs, adding to the
+  experimental `meld-3` tool.
+
 ### Fixed bugs
 
 * On Windows, symlinks in the repo are now materialized as regular files in the

cli/src/config/merge_tools.toml

@@ -11,6 +11,14 @@ merge-args = ["$left", "$base", "$right", "-o", "$output", "--auto-merge"]
 program="meld"
 # If using this as a template, note that `$output` is repeated twice below
 edit-args = ["$left", "$output", "$right", "-o", "$output"]
+# With the normal Meld merge conflict resolution setup, changes from `$base` to
+# either side are not highlighted. Here we offer an alternative that opens the
+# original two sides compared to the base in two more tabs.
+merge-args = ["--diff", "$left", "$output", "$right", "--diff", "$base", "$left", "--diff", "$base", "$right"]
+# The output pane in the first tab must be prepopulated with _something_. The
+# conflict markers were chosen here, rather than simply the base, because they
+# contain enough information to let you skip the other tabs for simple cases.
+merge-tool-edits-conflict-markers = true
 
 [merge-tools.vimdiff]
 program = "vim"

docs/config.md

@@ -521,6 +521,16 @@ the conflict is done, `jj` assumes that the conflict was only partially resolved
 and parses the conflict markers to get the new state of the conflict. The
 conflict is considered fully resolved when there are no conflict markers left.
 
+### Experimental 3-tab conflict resolution
+
+The `"meld-3"` merge editor opens 3 tabs in Meld. The first tab is a 3-pane
+view of the left and right sides with an editing pane in the middle. The
+editing pane in the middle of the first tab contains [enough
+information](conflicts.md#conflict-markers) on its own to resolve conflicts.
+The next two tabs are 2-pane views for reference, each showing a diff from the
+base to one of the sides. Of the two reference tabs, the last tab (the right
+side compared to base) is generally the more useful one.
+
 ## Commit Signing
 
 `jj` can be configured to sign and verify the commits it creates using either