patch 8.1.2289: after :diffsplit closing the window does not disable diff
Problem: After :diffsplit closing the window does not disable diff.
Solution: Add "closeoff" to 'diffopt' and add it to the default.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 8ad86ad..6f4eaaf 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -964,3 +964,28 @@
call StopVimInTerminal(buf)
call delete('Xtest_diff_diff')
endfunc
+
+func CloseoffSetup()
+ enew
+ call setline(1, ['one', 'two', 'three'])
+ diffthis
+ new
+ call setline(1, ['one', 'tow', 'three'])
+ diffthis
+ call assert_equal(1, &diff)
+ only!
+endfunc
+
+func Test_diff_closeoff()
+ " "closeoff" included by default: last diff win gets 'diff' reset'
+ call CloseoffSetup()
+ call assert_equal(0, &diff)
+ enew!
+
+ " "closeoff" excluded: last diff win keeps 'diff' set'
+ set diffopt-=closeoff
+ call CloseoffSetup()
+ call assert_equal(1, &diff)
+ diffoff!
+ enew!
+endfunc