fix(git): resolve sidebar and diff-split rendering bugs

lua/git/sidebar.lua

@@ -34,6 +34,7 @@ local SIDEBAR_WIDTH = 50
 ---@field worktree string
 ---@field lines table<integer, ow.Git.SidebarEntry>
 ---@field sidebar_win integer?
+---@field invocation_win integer?  window focused when the sidebar opened. The first diff repurposes it as the right pane
 ---@field diff_left_win integer?
 ---@field diff_right_win integer?
 ---@field user_aucmd integer?
@@ -575,39 +576,30 @@ local function reset_diff_win(win)
     end)
 end
 
----@param sidebar_win integer
+---Validate the window the user was in when the sidebar opened. The first
+---diff repurposes it as the right pane, regardless of whether it holds an
+---empty buffer or a real file. Returns nil if the user closed it, moved
+---to another tabpage, or it's somehow the sidebar itself.
+---@param s ow.Git.SidebarState
 ---@return integer?
-local function find_default_main_win(sidebar_win)
-    local non_sidebar = {}
-    for _, win in ipairs(vim.api.nvim_tabpage_list_wins(0)) do
-        if win ~= sidebar_win then
-            table.insert(non_sidebar, win)
-        end
-    end
-    if #non_sidebar ~= 1 then
+local function invocation_win_for(s)
+    local win = s.invocation_win
+    if not win or not vim.api.nvim_win_is_valid(win) then
         return nil
     end
-    local buf = vim.api.nvim_win_get_buf(non_sidebar[1])
-    if
-        vim.api.nvim_buf_get_name(buf) == ""
-        and vim.bo[buf].buftype == ""
-        and not vim.bo[buf].modified
-        and vim.api.nvim_buf_line_count(buf) == 1
-        and vim.api.nvim_buf_get_lines(buf, 0, 1, false)[1] == ""
-    then
-        return non_sidebar[1]
+    if win == s.sidebar_win then
+        return nil
     end
+    if
+        vim.api.nvim_win_get_tabpage(win)
+        ~= vim.api.nvim_get_current_tabpage()
+    then
+        return nil
+    end
+    return win
 end
 
----@param win integer
----@param enabled boolean
-local function set_diff(win, enabled)
-    vim.wo[win].diff = enabled
-    if enabled then
-        vim.wo[win].foldenable = true
-        vim.wo[win].foldlevel = 0
-    end
-end
+local set_diff = diff.set_diff
 
 ---@param s ow.Git.SidebarState
 ---@param sidebar_win integer
@@ -696,18 +688,19 @@ local function show_diff(s, entry, focus_left)
         left_win = vsplit_at(right_win, "left")
         reset_diff_win(left_win)
     elseif not (left_win or right_win) then
-        local default_main = find_default_main_win(sidebar_win)
-        if default_main then
-            right_win = default_main
+        local target = invocation_win_for(s)
+        if target then
+            right_win = target
             reset_diff_win(right_win)
             left_win = vsplit_at(right_win, "left")
             reset_diff_win(left_win)
         else
-            -- No reusable default-empty window. Open the diff pair by
-            -- splitting from the sidebar. winfixwidth keeps the sidebar at 50
-            -- when there are other windows to absorb the split; if the
-            -- sidebar is the only window in the tab, the split has to take
-            -- from the sidebar itself, so restore the width explicitly.
+            -- Invocation window is gone (closed or in another tabpage).
+            -- Open the diff pair by splitting from the sidebar. winfixwidth
+            -- keeps the sidebar at 50 when there are other windows to
+            -- absorb the split; if the sidebar is the only window in the
+            -- tab, the split has to take from the sidebar itself, so
+            -- restore the width explicitly.
             right_win = vsplit_at(sidebar_win, "right")
             reset_diff_win(right_win)
             left_win = vsplit_at(right_win, "left")
@@ -725,6 +718,13 @@ local function show_diff(s, entry, focus_left)
     s.diff_left_win = left_win
     s.diff_right_win = right_win
 
+    -- Toggle diff off around the buffer swap so Vim tears down the old
+    -- diff group and re-establishes a fresh one against the new pair.
+    -- nvim_win_set_buf swaps the buffer pointer without invalidating
+    -- cached diff state, and :diffupdate alone doesn't reliably force a
+    -- recompute when no buffer contents have actually changed.
+    set_diff(left_win, false)
+    set_diff(right_win, false)
     vim.api.nvim_win_set_buf(left_win, pair.left.buf)
     vim.api.nvim_win_set_buf(right_win, pair.right.buf)
     for _, side in ipairs({ pair.left, pair.right }) do
@@ -732,6 +732,15 @@ local function show_diff(s, entry, focus_left)
             diff.set_buf_name_and_filetype(side.buf, side.name)
         end
     end
+    -- Synthetic index/HEAD buffers never run BufRead, so modeline-only
+    -- filetypes aren't detected. Copy from the side that did resolve.
+    local left_ft = vim.bo[pair.left.buf].filetype
+    local right_ft = vim.bo[pair.right.buf].filetype
+    if left_ft == "" and right_ft ~= "" then
+        vim.bo[pair.left.buf].filetype = right_ft
+    elseif right_ft == "" and left_ft ~= "" then
+        vim.bo[pair.right.buf].filetype = left_ft
+    end
     set_diff(left_win, true)
     set_diff(right_win, true)
     s.last_shown_key = key
@@ -897,6 +906,7 @@ local function open(worktree)
         worktree = worktree,
         lines = {},
         sidebar_win = win,
+        invocation_win = previous_win,
     }
 
     local function k(lhs, rhs, desc)