patch 9.0.0661: multi-byte "lastline" in 'fillchars' does not work properly

Problem:    Multi-byte "lastline" item in 'fillchars' does not work properly
            when the window is two columns wide.
Solution:   Compute the text length correctly. (closes #11280)
diff --git a/src/testdir/test_display.vim b/src/testdir/test_display.vim
index 4d2ee9f..faed0b5 100644
--- a/src/testdir/test_display.vim
+++ b/src/testdir/test_display.vim
@@ -419,6 +419,10 @@
   call term_sendkeys(buf, ":3split\<CR>")
   call VerifyScreenDump(buf, $'Test_display_lastline_{a:euro}5', {})
 
+  call term_sendkeys(buf, ":close\<CR>")
+  call term_sendkeys(buf, ":2vsplit\<CR>")
+  call VerifyScreenDump(buf, $'Test_display_lastline_{a:euro}6', {})
+
   call StopVimInTerminal(buf)
 endfunc