patch 7.4.857
Problem:    Dragging the current tab with the mouse doesn't work properly.
Solution:   Take the current tabpage index into account. (Hirohito Higashi)
diff --git a/src/normal.c b/src/normal.c
index ae7b83a..69b0dce 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -2561,7 +2561,8 @@
 	    if (in_tab_line)
 	    {
 		c1 = TabPageIdxs[mouse_col];
-		tabpage_move(c1 <= 0 ? 9999 : c1 - 1);
+		tabpage_move(c1 <= 0 ? 9999 : c1 < tabpage_index(curtab)
+								? c1 - 1 : c1);
 	    }
 	    return FALSE;
 	}