patch 8.1.2308: deleting text before zero-width textprop removes it

Problem:    Deleting text before zero-width textprop removes it.
Solution:   Keep zero-width textprop when deleting text.

src/testdir/test_textprop.vim

@@ -650,6 +650,35 @@ func Test_prop_undo()
   call prop_type_delete('comment')
 endfunc
 
+func Test_prop_delete_text()
+  new
+  call prop_type_add('comment', {'highlight': 'Directory'})
+  call setline(1, ['oneone', 'twotwo', 'three'])
+
+  " zero length property
+  call prop_add(1, 3, {'type': 'comment'})
+  let expected = [{'col': 3, 'length': 0, 'id': 0, 'type': 'comment', 'start': 1, 'end': 1} ]
+  call assert_equal(expected, prop_list(1))
+
+  " delete one char moves the property
+  normal! x
+  let expected = [{'col': 2, 'length': 0, 'id': 0, 'type': 'comment', 'start': 1, 'end': 1} ]
+  call assert_equal(expected, prop_list(1))
+
+  " delete char of the property has no effect
+  normal! lx
+  let expected = [{'col': 2, 'length': 0, 'id': 0, 'type': 'comment', 'start': 1, 'end': 1} ]
+  call assert_equal(expected, prop_list(1))
+
+  " delete more chars moves property to first column, is not deleted
+  normal! 0xxxx
+  let expected = [{'col': 1, 'length': 0, 'id': 0, 'type': 'comment', 'start': 1, 'end': 1} ]
+  call assert_equal(expected, prop_list(1))
+
+  bwipe!
+  call prop_type_delete('comment')
+endfunc
+
 " screenshot test with textprop highlighting
 func Test_textprop_screenshot_various()
   CheckScreendump

src/textprop.c

@@ -1075,10 +1075,13 @@ adjust_prop_columns(
 	}
 	else if (bytes_added <= 0 && (tmp_prop.tp_col > col + 1))
 	{
+	    int len_changed = FALSE;
+
 	    if (tmp_prop.tp_col + bytes_added < col + 1)
 	    {
 		tmp_prop.tp_len += (tmp_prop.tp_col - 1 - col) + bytes_added;
 		tmp_prop.tp_col = col + 1;
+		len_changed = TRUE;
 	    }
 	    else
 		tmp_prop.tp_col += bytes_added;
@@ -1086,7 +1089,7 @@ adjust_prop_columns(
 	    if ((flags & APC_SAVE_FOR_UNDO) && !dirty)
 		u_savesub(lnum);
 	    dirty = TRUE;
-	    if (tmp_prop.tp_len <= 0)
+	    if (len_changed && tmp_prop.tp_len <= 0)
 		continue;  // drop this text property
 	}
 	else if (tmp_prop.tp_len > 0

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2308,
 /**/
     2307,
 /**/