patch 9.0.0915: WinScrolled may trigger immediately when defined

Problem:    WinScrolled may trigger immediately when defined.
Solution:   Initialize the fields in all windows. (closes #11582)

src/testdir/dumps/Test_winscrolled_not_when_defined_1.dump

@@ -0,0 +1,10 @@
+>a+0&#ffffff0@2| @56
+|b@2| @56
+|~+0#4040ff13&| @58
+|~| @58
+|[+3#0000000&|N|o| |N|a|m|e|]| |[|+|]| @28|1|,|1| @11|A|l@1
+|a+0&&@2| @56
+|b@2| @56
+|~+0#4040ff13&| @58
+|[+1#0000000&|N|o| |N|a|m|e|]| |[|+|]| @28|1|,|1| @11|A|l@1
+|:+0&&|a|u| |W|i|n|S|c|r|o|l@1|e|d| |*| |c|a|l@1| |t|i|m|e|r|_|s|t|a|r|t|(|1|0@1|,| |'|S|h|o|w|T|r|i|g@1|e|r|e|d|'|)| @3

src/testdir/dumps/Test_winscrolled_not_when_defined_2.dump

@@ -0,0 +1,10 @@
+>b+0&#ffffff0@2| @56
+|~+0#4040ff13&| @58
+|~| @58
+|~| @58
+|[+3#0000000&|N|o| |N|a|m|e|]| |[|+|]| @28|2|,|1| @11|B|o|t
+|a+0&&@2| @56
+|b@2| @56
+|~+0#4040ff13&| @58
+|[+1#0000000&|N|o| |N|a|m|e|]| |[|+|]| @28|1|,|1| @11|A|l@1
+|t+0&&|r|i|g@1|e|r|e|d| @50

src/testdir/test_autocmd.vim

@@ -439,6 +439,32 @@ func Test_WinScrolled_once_only()
   call StopVimInTerminal(buf)
 endfunc
 
+" Check that WinScrolled is not triggered immediately when defined and there
+" are split windows.
+func Test_WinScrolled_not_when_defined()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+      call setline(1, ['aaa', 'bbb'])
+      echo 'nothing happened'
+      func ShowTriggered(id)
+        echo 'triggered'
+      endfunc
+  END
+  call writefile(lines, 'Xtest_winscrolled_not', 'D')
+  let buf = RunVimInTerminal('-S Xtest_winscrolled_not', #{rows: 10, cols: 60, statusoff: 2})
+  call term_sendkeys(buf, ":split\<CR>")
+  call TermWait(buf)
+  " use a timer to show the message after redrawing
+  call term_sendkeys(buf, ":au WinScrolled * call timer_start(100, 'ShowTriggered')\<CR>")
+  call VerifyScreenDump(buf, 'Test_winscrolled_not_when_defined_1', {})
+
+  call term_sendkeys(buf, "\<C-E>")
+  call VerifyScreenDump(buf, 'Test_winscrolled_not_when_defined_2', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_WinScrolled_long_wrapped()
   CheckRunVimInTerminal