fix(column): don't count signs on lines beyond eob #33410

Problem:  Computed previous buffer line count may be beyond end of
          buffer. This results in signs being removed from `b_signcols`
          that were never included in it, tripping an assertion.

Solution: Store the previous line count as it was before appending or
          deleting lines. Use it to clamp the edited region when
          clearing signs before a splice, after which it is reset.
(cherry picked from commit 4a706a7092)

src/nvim/buffer_defs.h

@@ -718,6 +718,11 @@ struct file_buffer {
   MarkTree b_marktree[1];
   Map(uint32_t, uint32_t) b_extmark_ns[1];         // extmark namespaces
 
+  // Store the line count as it was before appending or inserting lines.
+  // Used to determine a valid range before splicing marks, when the line
+  // count has already changed.
+  int b_prev_line_count;
+
   // array of channel_id:s which have asked to receive updates for this
   // buffer.
   kvec_t(uint64_t) update_channels;

src/nvim/extmark.c

@@ -580,8 +580,9 @@ void extmark_splice_impl(buf_T *buf, int start_row, colnr_T start_col, bcount_t
 
   // Remove signs inside edited region from "b_signcols.count", add after splicing.
   if (old_row > 0 || new_row > 0) {
-    int row2 = MIN(buf->b_ml.ml_line_count - (new_row - old_row) - 1, start_row + old_row);
-    buf_signcols_count_range(buf, start_row, row2, 0, kTrue);
+    int count = buf->b_prev_line_count > 0 ? buf->b_prev_line_count : buf->b_ml.ml_line_count;
+    buf_signcols_count_range(buf, start_row, MIN(count - 1, start_row + old_row), 0, kTrue);
+    buf->b_prev_line_count = 0;
   }
 
   marktree_splice(buf->b_marktree, (int32_t)start_row, start_col,

src/nvim/memline.c

@@ -2088,6 +2088,9 @@ static int ml_append_int(buf_T *buf, linenr_T lnum, char *line, colnr_T len, boo
     dp = hp->bh_data;
   }
 
+  if (buf->b_prev_line_count == 0) {
+    buf->b_prev_line_count = buf->b_ml.ml_line_count;
+  }
   buf->b_ml.ml_line_count++;
 
   if ((int)dp->db_free >= space_needed) {       // enough room in data block
@@ -2573,6 +2576,9 @@ static int ml_delete_int(buf_T *buf, linenr_T lnum, bool message)
   int count = buf->b_ml.ml_locked_high - buf->b_ml.ml_locked_low + 2;
   int idx = lnum - buf->b_ml.ml_locked_low;
 
+  if (buf->b_prev_line_count == 0) {
+    buf->b_prev_line_count = buf->b_ml.ml_line_count;
+  }
   buf->b_ml.ml_line_count--;
 
   int line_start = ((dp->db_index[idx]) & DB_INDEX_MASK);

test/functional/ui/sign_spec.lua

@@ -681,4 +681,11 @@ describe('Signs', function()
       :lua log, needs_clear = {}, true                     |
     ]])
   end)
+
+  it('signcolumn tracking does not consider signs beyond eob', function()
+    api.nvim_set_option_value('signcolumn', 'auto:2', {})
+    api.nvim_buf_set_extmark(0, api.nvim_create_namespace(''), 1, 0, { sign_text = 's' })
+    api.nvim_buf_set_lines(0, 0, -1, false, {})
+    n.assert_alive()
+  end)
 end)