vim-patch:9.0.2022: getmousepos() returns wrong index for TAB char (#25636)

Problem:  When clicking in the middle of a TAB, getmousepos() returns
          the column of the next char instead of the TAB.
Solution: Break out of the loop when the vcol to find is inside current
          char. Fix invalid memory access when calling virtcol2col() on
          an empty line.

closes: vim/vim#13321

https://github.com/vim/vim/commit/b583eda7031b1f6a3469a2537d0c10ca5fa5568e

1 files changed, 8 insertions, 4 deletions

diff --git a/src/nvim/move.c b/src/nvim/move.c
index 25dee0a114..dfd2bf795d 100644
--- a/src/nvim/move.c
+++ b/src/nvim/move.c
@@ -1142,13 +1142,17 @@ void f_screenpos(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 /// returned.
 static int virtcol2col(win_T *wp, linenr_T lnum, int vcol)
 {
-  int offset = vcol2col(wp, lnum, vcol);
+  int offset = vcol2col(wp, lnum, vcol - 1);
   char *line = ml_get_buf(wp->w_buffer, lnum);
   char *p = line + offset;
 
-  // For a multibyte character, need to return the column number of the first byte.
-  MB_PTR_BACK(line, p);
-
+  if (*p == NUL) {
+    if (p == line) {  // empty line
+      return 0;
+    }
+    // Move to the first byte of the last char.
+    MB_PTR_BACK(line, p);
+  }
   return (int)(p - line + 1);
 }