Fix missing syntax highlighting in proposed diff view

Makefile

@@ -3,12 +3,19 @@
 # Default target
 all: format check test
 
+# Detect if we are already inside a Nix shell
+ifeq (,$(IN_NIX_SHELL))
+NIX_PREFIX := nix develop .#ci -c
+else
+NIX_PREFIX :=
+endif
+
 # Check for syntax errors
 check:
 	@echo "Checking Lua files for syntax errors..."
-	nix develop .#ci -c find lua -name "*.lua" -type f -exec lua -e "assert(loadfile('{}'))" \;
+	$(NIX_PREFIX) find lua -name "*.lua" -type f -exec lua -e "assert(loadfile('{}'))" \;
 	@echo "Running luacheck..."
-	nix develop .#ci -c luacheck lua/ tests/ --no-unused-args --no-max-line-length
+	$(NIX_PREFIX) luacheck lua/ tests/ --no-unused-args --no-max-line-length
 
 # Format all files
 format:

lua/claudecode/diff.lua

@@ -225,12 +225,57 @@ function M._cleanup_diff_layout(tab_name, target_win, new_win)
   logger.debug("diff", "[CLEANUP] Layout cleanup completed for:", tab_name)
 end
 
+-- Detect filetype from a path or existing buffer (best-effort)
+local function _detect_filetype(path, buf)
+  -- 1) Try Neovim's builtin matcher if available (>=0.10)
+  if vim.filetype and type(vim.filetype.match) == "function" then
+    local ok, ft = pcall(vim.filetype.match, { filename = path })
+    if ok and ft and ft ~= "" then
+      return ft
+    end
+  end
+
+  -- 2) Try reading from existing buffer
+  if buf and vim.api.nvim_buf_is_valid(buf) then
+    local ft = vim.api.nvim_buf_get_option(buf, "filetype")
+    if ft and ft ~= "" then
+      return ft
+    end
+  end
+
+  -- 3) Fallback to simple extension mapping
+  local ext = path:match("%.([%w_%-]+)$") or ""
+  local simple_map = {
+    lua = "lua",
+    ts = "typescript",
+    js = "javascript",
+    jsx = "javascriptreact",
+    tsx = "typescriptreact",
+    py = "python",
+    go = "go",
+    rs = "rust",
+    c = "c",
+    h = "c",
+    cpp = "cpp",
+    hpp = "cpp",
+    md = "markdown",
+    sh = "sh",
+    zsh = "zsh",
+    bash = "bash",
+    json = "json",
+    yaml = "yaml",
+    yml = "yaml",
+    toml = "toml",
+  }
+  return simple_map[ext]
+end
 --- Open diff using native Neovim functionality
 -- @param old_file_path string Path to the original file
 -- @param new_file_path string Path to the new file (used for naming)
 -- @param new_file_contents string Contents of the new file
 -- @param tab_name string Name for the diff tab/view
 -- @return table Result with provider, tab_name, and success status
+
 function M._open_native_diff(old_file_path, new_file_path, new_file_contents, tab_name)
   local new_filename = vim.fn.fnamemodify(new_file_path, ":t") .. ".new"
   local tmp_file, err = M._create_temp_file(new_file_contents, new_filename)
@@ -259,9 +304,16 @@ function M._open_native_diff(old_file_path, new_file_path, new_file_contents, ta
   vim.cmd("edit " .. vim.fn.fnameescape(tmp_file))
   vim.api.nvim_buf_set_name(0, new_file_path .. " (New)")
 
+  -- Propagate filetype to the proposed buffer for proper syntax highlighting (#20)
+  local proposed_buf = vim.api.nvim_get_current_buf()
+  local old_filetype = _detect_filetype(old_file_path)
+  if old_filetype and old_filetype ~= "" then
+    vim.api.nvim_set_option_value("filetype", old_filetype, { buf = proposed_buf })
+  end
+
   vim.cmd("wincmd =")
 
-  local new_buf = vim.api.nvim_get_current_buf()
+  local new_buf = proposed_buf
   vim.api.nvim_set_option_value("buftype", "nofile", { buf = new_buf })
   vim.api.nvim_set_option_value("bufhidden", "wipe", { buf = new_buf })
   vim.api.nvim_set_option_value("swapfile", false, { buf = new_buf })
@@ -665,6 +717,12 @@ function M._create_diff_view_from_window(target_window, old_file_path, new_buffe
   vim.cmd("vsplit")
   local new_win = vim.api.nvim_get_current_win()
   vim.api.nvim_win_set_buf(new_win, new_buffer)
+
+  -- Ensure new buffer inherits filetype from original for syntax highlighting (#20)
+  local original_ft = _detect_filetype(old_file_path, original_buffer)
+  if original_ft and original_ft ~= "" then
+    vim.api.nvim_set_option_value("filetype", original_ft, { buf = new_buffer })
+  end
   vim.cmd("diffthis")
   logger.debug("diff", "Created split window", new_win, "with new buffer", new_buffer)
 

tests/unit/diff_spec.lua

@@ -215,6 +215,40 @@ describe("Diff Module", function()
     end)
   end)
 
+  describe("Filetype Propagation", function()
+    it("should propagate original filetype to proposed buffer", function()
+      diff.setup({})
+
+      -- Spy on nvim_set_option_value
+      spy.on(_G.vim.api, "nvim_set_option_value")
+
+      local mock_file = {
+        write = function() end,
+        close = function() end,
+      }
+      local old_io_open = io.open
+      rawset(io, "open", function()
+        return mock_file
+      end)
+
+      local result = diff._open_native_diff("/tmp/test.ts", "/tmp/test.ts", "-- new", "Propagate FT")
+      expect(result.success).to_be_true()
+
+      -- Verify spy called with filetype typescript
+      local calls = _G.vim.api.nvim_set_option_value.calls or {}
+      local found = false
+      for _, c in ipairs(calls) do
+        if c.vals[1] == "filetype" and c.vals[2] == "typescript" then
+          found = true
+          break
+        end
+      end
+      expect(found).to_be_true()
+
+      rawset(io, "open", old_io_open)
+    end)
+  end)
+
   describe("Open Diff Function", function()
     it("should use native provider", function()
       diff.setup({})