refactor(git): rebuild status_view diff layout per selection

test/git/status_view_test.lua

@@ -39,16 +39,28 @@ local function find_sidebar()
     end
 end
 
----Find a diff-role window in the given tabpage (or current).
+---Find a diff window in the given tabpage (or current). "left" / "right"
+---is determined by column position: the layout is [sidebar | left | right],
+---so the leftmost &diff window is the left pane and the rightmost is the
+---right pane.
 ---@param role "left"|"right"
 ---@param tab integer?
 ---@return integer?
 local function find_diff_win(role, tab)
+    local diffs = {}
     for _, w in ipairs(vim.api.nvim_tabpage_list_wins(tab or 0)) do
-        if vim.w[w].git_diff_role == role then
-            return w
+        if vim.wo[w].diff then
+            table.insert(diffs, w)
         end
     end
+    table.sort(diffs, function(a, b)
+        return vim.api.nvim_win_get_position(a)[2]
+            < vim.api.nvim_win_get_position(b)[2]
+    end)
+    if role == "left" then
+        return diffs[1]
+    end
+    return diffs[#diffs]
 end
 
 ---@param file_path string
@@ -161,7 +173,6 @@ t.test(
             setup_sidebar_with_unstaged_file("foo.txt", "v1\n", "v2\n")
         local tab1 = vim.api.nvim_get_current_tabpage()
 
-        -- First show diff in tab1, so state.diff_*_win point at tab1.
         vim.api.nvim_set_current_win(sidebar_win)
         vim.api.nvim_win_set_cursor(sidebar_win, { line, 0 })
         t.press("<Tab>")