3 files changed, 16 insertions, 6 deletions
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 14476e29d4..6ba5a171f9 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -12107,6 +12107,8 @@ M.funcs = {
       character in window {winid} at buffer line {lnum} and virtual
       column {col}.
 
+      If buffer line {lnum} is an empty line, 0 is returned.
+
       If {col} is greater than the last virtual column in line
       {lnum}, then the byte index of the character at the last
       virtual column is returned.
diff --git a/src/nvim/mouse.c b/src/nvim/mouse.c
index 0433031393..75c399bcad 100644
--- a/src/nvim/mouse.c
+++ b/src/nvim/mouse.c
@@ -1754,7 +1754,7 @@ static win_T *mouse_find_grid_win(int *gridp, int *rowp, int *colp)
 }
 
 /// Convert a virtual (screen) column to a character column.
-/// The first column is one.
+/// The first column is zero.
 colnr_T vcol2col(win_T *const wp, const linenr_T lnum, const colnr_T vcol)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
@@ -1763,7 +1763,11 @@ colnr_T vcol2col(win_T *const wp, const linenr_T lnum, const colnr_T vcol)
   chartabsize_T cts;
   init_chartabsize_arg(&cts, wp, lnum, 0, line, line);
   while (cts.cts_vcol < vcol && *cts.cts_ptr != NUL) {
-    cts.cts_vcol += win_lbr_chartabsize(&cts, NULL);
+    int size = win_lbr_chartabsize(&cts, NULL);
+    if (cts.cts_vcol + size > vcol) {
+      break;
+    }
+    cts.cts_vcol += size;
     MB_PTR_ADV(cts.cts_ptr);
   }
   clear_chartabsize_arg(&cts);
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);
 }