patch 8.0.1237: ":set scroll&" often gives an error

Problem:    ":set scroll&" often gives an error.
Solution:   Don't use a fixed default value, use half the window height. Add a
            test. (Ozaki Kiichi, closes #2104)

src/Makefile

@@ -2242,6 +2242,7 @@ test_arglist \
 	test_reltime \
 	test_retab \
 	test_ruby \
+	test_scroll_opt \
 	test_scrollbind \
 	test_search \
 	test_searchpos \

src/option.c

@@ -2359,7 +2359,7 @@ static struct vimoption options[] =
 			    SCRIPTID_INIT},
     {"scroll",	    "scr",  P_NUM|P_NO_MKRC|P_VI_DEF,
 			    (char_u *)VAR_WIN, PV_SCROLL,
-			    {(char_u *)12L, (char_u *)0L} SCRIPTID_INIT},
+			    {(char_u *)0L, (char_u *)0L} SCRIPTID_INIT},
     {"scrollbind",  "scb",  P_BOOL|P_VI_DEF,
 #ifdef FEAT_SCROLLBIND
 			    (char_u *)VAR_WIN, PV_SCBIND,
@@ -3904,10 +3904,9 @@ set_init_2(void)
     int		idx;
 
     /*
-     * 'scroll' defaults to half the window height. Note that this default is
+     * 'scroll' defaults to half the window height. The stored default is zero,
-     * wrong when the window height changes.
+     * which results in the actual value computed from the window height.
      */
-    set_number_default("scroll", (long)((long_u)Rows >> 1));
     idx = findoption((char_u *)"scroll");
     if (idx >= 0 && !(options[idx].flags & P_WAS_SET))
 	set_option_default(idx, OPT_LOCAL, p_cp);

src/testdir/test_alot.vim

@@ -41,6 +41,7 @@ source test_popup.vim
 source test_put.vim
 source test_recover.vim
 source test_reltime.vim
+source test_scroll_opt.vim
 source test_searchpos.vim
 source test_set.vim
 source test_sort.vim

src/testdir/test_scroll_opt.vim

@@ -0,0 +1,36 @@
+" Test for reset 'scroll'
+"
+
+func Test_reset_scroll()
+  let scr = &l:scroll
+
+  setlocal scroll=1
+  setlocal scroll&
+  call assert_equal(scr, &l:scroll)
+
+  setlocal scroll=1
+  setlocal scroll=0
+  call assert_equal(scr, &l:scroll)
+
+  try
+    execute 'setlocal scroll=' . (winheight(0) + 1)
+    " not reached
+    call assert_false(1)
+  catch
+    call assert_exception('E49:')
+  endtry
+
+  split
+
+  let scr = &l:scroll
+
+  setlocal scroll=1
+  setlocal scroll&
+  call assert_equal(scr, &l:scroll)
+
+  setlocal scroll=1
+  setlocal scroll=0
+  call assert_equal(scr, &l:scroll)
+
+  quit!
+endfunc

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1237,
 /**/
     1236,
 /**/