feat(merge): add conflict_choose_side action to replace merged buffer with one whole side

doc/diffview.txt

@@ -133,6 +133,16 @@ COMMANDS                                        *diffview-commands*
           • `dx`: Choose none of the versions of the conflict (delete the
             conflict region).
 
+        For operating on the merged file as a whole (useful when the merge
+        driver did not emit conflict markers, e.g., modify/delete conflicts
+        or jj with `merge-tool-edits-conflict-markers = false`):
+          • `<leader>cso`: Replace the merged buffer with the entire OURS
+            side.
+          • `<leader>cst`: Replace the merged buffer with the entire THEIRS
+            side.
+          • `<leader>csb`: Replace the merged buffer with the entire BASE
+            side.
+
         For more info on these actions, see
         |diffview-actions-conflict_choose|.
 
@@ -1717,6 +1727,22 @@ conflict_choose_all({version})                  *diffview-actions-conflict_choos
 
         Parameters: ~
             {version} `'ours'|'theirs'|'base'|'all'|'none'`
+                See |diffview-conflict-versions| to tell what versions of the
+                file these names correspond with.
+
+conflict_choose_side({version})                 *diffview-actions-conflict_choose_side*
+        Contexts: `merge_tool`, `file_panel`
+
+        Replace the entire merged buffer with the content of one whole side
+        (OURS, THEIRS, or BASE). Unlike |diffview-actions-conflict_choose_all|,
+        this does not parse conflict markers; it overwrites the buffer with
+        the side's lines verbatim. Useful when the merge driver did not emit
+        conflict markers (e.g., modify/delete conflicts, or jj with
+        `merge-tool-edits-conflict-markers = false`); accepting a side whose
+        path was empty effectively accepts the delete.
+
+        Parameters: ~
+            {version} `'ours'|'theirs'|'base'`
                 See |diffview-conflict-versions| to tell what versions of the
                 file these names correspond with.
 

lua/diffview/actions.lua

@@ -75,6 +75,7 @@ local pl = lazy.access(utils, "path") --[[@as PathLib ]]
 ---@field unstage_all fun()
 ---@field conflict_choose fun(target: DiffviewConflictTarget): fun()
 ---@field conflict_choose_all fun(target: DiffviewConflictTarget): AsyncFunc
+---@field conflict_choose_side fun(target: DiffviewConflictSideTarget): AsyncFunc
 ---@field cycle_layout fun()
 ---@field diff_against_default_branch fun()
 ---@field diffget fun(target: DiffviewDiffgetTarget): fun()
@@ -766,6 +767,55 @@ function M.conflict_choose(target)
   end, "merge_only")
 end
 
+---Replace the entire MERGED buffer with the content of the OURS, THEIRS, or
+---BASE side. Unlike `conflict_choose_all`, this does not parse conflict
+---markers; it overwrites the whole buffer with the side's lines. Useful when
+---the merge driver did not emit markers (e.g., modify/delete conflicts, or
+---jj with `merge-tool-edits-conflict-markers = false`).
+---@param target DiffviewConflictSideTarget
+---@return AsyncFunc
+function M.conflict_choose_side(target)
+  return tag(
+    async.void(function()
+      local view = lib.get_current_view() --[[@as DiffView ]]
+
+      if not (view and view:instanceof(DiffView.__get())) then
+        return
+      end
+      ---@cast view DiffView
+
+      if view.panel:is_focused() then
+        local item = view:infer_cur_file(false) ---@cast item -DirData
+        if not item then
+          return
+        end
+
+        if not item.active then
+          await(view:set_file(item))
+        end
+      end
+
+      local main, bufnr = get_valid_main(view)
+      if not main then
+        return
+      end
+      ---@cast bufnr integer
+
+      local src_bufnr = diff_copy_target(target)
+      if not (src_bufnr and api.nvim_buf_is_valid(src_bufnr)) then
+        return
+      end
+
+      local src_lines = api.nvim_buf_get_lines(src_bufnr, 0, -1, false)
+      api.nvim_buf_set_lines(bufnr, 0, -1, false, src_lines)
+
+      utils.set_cursor(main.id, 1, 0)
+      view.cur_layout:sync_scroll()
+    end),
+    "merge_only"
+  )
+end
+
 ---@param target DiffviewDiffgetTarget
 ---@return fun()
 function M.diffget(target)

lua/diffview/config.lua

@@ -50,6 +50,7 @@ end
 
 -- Targets consumed by action factories in `actions.lua` (referenced from keymaps).
 ---@alias DiffviewConflictTarget "ours"|"theirs"|"base"|"all"|"none"
+---@alias DiffviewConflictSideTarget "ours"|"theirs"|"base"
 ---@alias DiffviewDiffgetTarget "ours"|"theirs"|"base"|"local"
 
 ---@class DiffviewKeymapOpts
@@ -120,6 +121,9 @@ local conflict_keymaps = {
   { "n", "<leader>cB",  actions.conflict_choose_all("base"),    { desc = "Choose the BASE version of a conflict for the whole file" } },
   { "n", "<leader>cA",  actions.conflict_choose_all("all"),     { desc = "Choose all the versions of a conflict for the whole file" } },
   { "n", "dX",          actions.conflict_choose_all("none"),    { desc = "Delete the conflict region for the whole file" } },
+  { "n", "<leader>cso", actions.conflict_choose_side("ours"),   { desc = "Replace the MERGED buffer with the entire OURS side" } },
+  { "n", "<leader>cst", actions.conflict_choose_side("theirs"), { desc = "Replace the MERGED buffer with the entire THEIRS side" } },
+  { "n", "<leader>csb", actions.conflict_choose_side("base"),   { desc = "Replace the MERGED buffer with the entire BASE side" } },
 }
 
 ---@class DiffviewConfig

lua/diffview/tests/functional/actions_spec.lua

@@ -279,6 +279,12 @@ describe("diffview.actions._is_applicable", function()
     assert.is_true(actions._is_applicable(fn, make_view({ merge_ctx = true })))
   end)
 
+  it("propagates `merge_only` tag through `conflict_choose_side` factory", function()
+    local fn = actions.conflict_choose_side("ours")
+    assert.is_false(actions._is_applicable(fn, make_view()))
+    assert.is_true(actions._is_applicable(fn, make_view({ merge_ctx = true })))
+  end)
+
   it("returns false when view is nil and the action is `merge_only`", function()
     assert.is_false(actions._is_applicable(actions.next_conflict, nil))
   end)