patch 9.2.0585: line number wrong after undoing a deletion in quickfix buffer

Problem:  Deleting a quickfix line and undoing it leaves the entry
          pointing one line below where it should.
Solution: Don't shift already cleared entries in qf_mark_adjust.

closes: #20379

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/quickfix.c

@@ -4434,6 +4434,9 @@ qf_mark_adjust(
 		if (qfp->qf_fnum == curbuf->b_fnum)
 		{
 		    found_one = TRUE;
+		    if (qfp->qf_cleared)
+			continue;
+
 		    if (qfp->qf_lnum >= line1 && qfp->qf_lnum <= line2)
 		    {
 			if (amount == MAXLNUM)

src/testdir/test_quickfix.vim

@@ -2334,6 +2334,53 @@ func Test_adjust_lnum()
   call Xadjust_qflnum('l')
 endfunc
 
+func Xqf_undo_after_delete(cchar)
+  call s:setup_commands(a:cchar)
+
+  enew | only
+
+  let fname = 'Xqfundofile' . a:cchar
+  call s:create_test_file(fname)
+  exe 'edit ' . fname
+
+  Xgetexpr [fname . ':5:Line5',
+	      \ fname . ':10:Line10',
+	      \ fname . ':15:Line15']
+
+  " Delete the line of the second error and undo the deletion.
+  10delete
+  let l = g:Xgetlist()
+  call assert_equal(5, l[0].lnum)
+  call assert_equal(10, l[1].lnum)
+  call assert_equal(14, l[2].lnum)
+
+  undo
+  call assert_equal('Line10', getline(10))
+  let l = g:Xgetlist()
+  call assert_equal(5, l[0].lnum)
+  call assert_equal(10, l[1].lnum)
+  call assert_equal(15, l[2].lnum)
+
+  " Redo and undo again to make sure the line number does not drift.
+  redo
+  call assert_equal(14, g:Xgetlist()[2].lnum)
+  undo
+  let l = g:Xgetlist()
+  call assert_equal(10, l[1].lnum)
+  call assert_equal(15, l[2].lnum)
+
+  call g:Xsetlist([], 'f')
+  enew!
+  call delete(fname)
+endfunc
+
+func Test_qf_undo_after_delete()
+  call setloclist(0, [])
+  call Xqf_undo_after_delete('c')
+  call setqflist([])
+  call Xqf_undo_after_delete('l')
+endfunc
+
 " Tests for the :grep/:lgrep and :grepadd/:lgrepadd commands
 func s:test_xgrep(cchar)
   call s:setup_commands(a:cchar)

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    585,
 /**/
     584,
 /**/