patch 8.0.0518: bad fold text when a multi-byte char has a zero byte

Problem:    Storing a zero byte from a multi-byte character causes fold text
            to show up wrong.
Solution:   Avoid putting zero in ScreenLines. (Christian Brabandt,
            closes #1567)

src/screen.c

@@ -2697,12 +2697,15 @@ fold_line(
 	    {
 		ScreenLinesUC[off + col] = fill_fold;
 		ScreenLinesC[0][off + col] = 0;
+                ScreenLines[off + col] = 0x80; /* avoid storing zero */
 	    }
 	    else
 		ScreenLinesUC[off + col] = 0;
+	    col++;
 	}
+	else
 #endif
-	ScreenLines[off + col++] = fill_fold;
+	    ScreenLines[off + col++] = fill_fold;
     }
 
     if (text != buf)

src/testdir/test_display.vim

@@ -3,18 +3,12 @@ if !has('gui_running') && has('unix')
   set term=ansi
 endif
 
-function! s:screenline(lnum, nr) abort
-  let line = []
-  for j in range(a:nr)
-    for c in range(1, winwidth(0))
-        call add(line, nr2char(screenchar(a:lnum+j, c)))
-    endfor
-    call add(line, "\n")
-  endfor
-  return join(line, '')
-endfunction
+source view_util.vim
 
-function! Test_display_foldcolumn()
+func! Test_display_foldcolumn()
+  if !has("folding")
+    return
+  endif
   new
   vnew
   vert resize 25
@@ -23,17 +17,43 @@ function! Test_display_foldcolumn()
 
   1put='e more noise blah blah‚ more stuff here'
 
-  let expect = "e more noise blah blah<82\n> more stuff here        \n"
+  let expect = [
+        \ "e more noise blah blah<82",
+        \ "> more stuff here        "
+        \ ]
 
   call cursor(2, 1)
   norm! zt
-  redraw!
-  call assert_equal(expect, s:screenline(1,2))
+  let lines=ScreenLines([1,2], winwidth(0))
+  call assert_equal(expect, lines)
   set fdc=2
-  redraw!
-  let expect = "  e more noise blah blah<\n  82> more stuff here    \n"
-  call assert_equal(expect, s:screenline(1,2))
+  let lines=ScreenLines([1,2], winwidth(0))
+  let expect = [
+        \ "  e more noise blah blah<",
+        \ "  82> more stuff here    "
+        \ ]
+  call assert_equal(expect, lines)
 
   quit!
   quit!
-endfunction
+endfunc
+
+func! Test_display_foldtext_mbyte()
+  if !has("folding") || !has("multi_byte")
+    return
+  endif
+  call NewWindow(10, 40)
+  call append(0, range(1,20))
+  exe "set foldmethod=manual foldtext=foldtext() fillchars=fold:\u2500,vert:\u2502 fdc=2"
+  call cursor(2, 1)
+  norm! zf13G
+  let lines=ScreenLines([1,3], winwidth(0)+1)
+  let expect=[
+        \ "  1                                     \u2502",
+        \ "+ +-- 12 lines: 2". repeat("\u2500", 23). "\u2502",
+        \ "  14                                    \u2502",
+        \ ]
+  call assert_equal(expect, lines)
+  set foldtext& fillchars& foldmethod& fdc&
+  bw!
+endfunc

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    518,
 /**/
     517,
 /**/