patch 9.1.0601: Wrong cursor position with 'breakindent' when wide char doesn't fit

Problem:  Wrong cursor position with 'breakindent' when a double-width
          character doesn't fit in a screen line (mikoto2000)
Solution: Include the width of the 'breakindent' properly.
          (zeertzjq)

fixes: #15289
closes: #15290

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_breakindent.vim b/src/testdir/test_breakindent.vim
index 96d91c9..b306c02 100644
--- a/src/testdir/test_breakindent.vim
+++ b/src/testdir/test_breakindent.vim
@@ -1165,4 +1165,15 @@
   call s:close_windows()
 endfunc
 
+func Test_breakindent_with_double_width_wrap()
+  50vnew
+  setlocal tabstop=8 breakindent nolist
+  call setline(1, "\t" .. repeat('a', winwidth(0) - 9) .. '口口口')
+  normal! $g0
+  call assert_equal(2, winline())
+  call assert_equal(9, wincol())
+
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab