From d73cfefed5e45fec91a422cd87054611ff711b30 Mon Sep 17 00:00:00 2001
From: tao <2471314@gmail.com>
Date: Fri, 22 Aug 2025 10:55:45 +0800
Subject: [PATCH] fix(folds): error when deleting lines at end of buffer #35396

Problem:
with `foldmethod=expr foldexpr=v:lua.vim.treesitter.foldexpr()
foldminlines=0`, deleting lines at the end of the buffer always
reports an invalid top error, because the top value (i.e. the
start line number of the deletion) is always 1 greater than
the total line number of the modified buffer.

Solution:
remove the ml_line_count validation
---
 src/nvim/lua/stdlib.c                    |  2 +-
 test/functional/treesitter/fold_spec.lua | 12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/nvim/lua/stdlib.c b/src/nvim/lua/stdlib.c
index 4a3b8596b3..a57ca059b7 100644
--- a/src/nvim/lua/stdlib.c
+++ b/src/nvim/lua/stdlib.c
@@ -563,7 +563,7 @@ static int nlua_foldupdate(lua_State *lstate)
   }
   // input is zero-based end-exclusive range
   linenr_T top = (linenr_T)luaL_checkinteger(lstate, 2) + 1;
-  if (top < 1 || top > win->w_buffer->b_ml.ml_line_count) {
+  if (top < 1) {
     return luaL_error(lstate, "invalid top");
   }
   linenr_T bot = (linenr_T)luaL_checkinteger(lstate, 3);
diff --git a/test/functional/treesitter/fold_spec.lua b/test/functional/treesitter/fold_spec.lua
index a050b60328..d4a96bc28e 100644
--- a/test/functional/treesitter/fold_spec.lua
+++ b/test/functional/treesitter/fold_spec.lua
@@ -832,4 +832,16 @@ t2]])
     command('set ft=c')
     eq(foldlevels, get_fold_levels())
   end)
+
+  it('no error when deleting lines at end of buffer with fml=0', function()
+    local screen = Screen.new(40, 2)
+    insert('hello')
+    parse('markdown')
+    command('set foldmethod=expr foldexpr=v:lua.vim.treesitter.foldexpr() foldminlines=0')
+    feed('o<Esc>dd')
+    screen:expect([[
+      ^hello                                   |
+                                              |
+    ]])
+  end)
 end)