Take account of borders when resizing floating panes.

layout.c

@@ -698,6 +698,8 @@ layout_resize_floating_pane_to(struct window_pane *wp, enum layout_type type,
 		return;
 	}
 
+	if (size >= PANE_MINIMUM + 2)
+		size -= 2;
 	if (size < PANE_MINIMUM || size > PANE_MAXIMUM) {
 		*cause = xstrdup("size is too big or too small");
 		return;