1 files changed, 4 insertions, 2 deletions

diff --git a/src/window.c b/src/window.c
index 5342f8b4869..db7d06056e3 100644
--- a/src/window.c
+++ b/src/window.c
@@ -2613,8 +2613,10 @@ SIZE includes that window's scroll bar, or the divider column to its right.")
     {
       if (!NILP (horflag))
         /* Calculate the size of the left-hand window, by dividing
-           the usable space in columns by two. */
-        size_int = XFASTINT (o->width) >> 1;
+           the usable space in columns by two.
+           We round up, since the left-hand window may include
+           a dividing line, while the right-hand may not.  */
+        size_int = (XFASTINT (o->width) + 1) >> 1;
       else
         size_int = XFASTINT (o->height) >> 1;
     }