Timetable: parallel session display with 'default room'


I came across a slightly annoying problem in the timetable: If you have parallel sessions and not yet assigned the final rooms (instead just use the checkbox for the global location) the layout is broken in the frontend view (all sessions are overlaid).

I already traced it down to the timetable js:
indico/web/client/js/legacy/libs/timetable/Draw.js around line 1013ff. where it calls getNumColumnsForGroup (from Layout.js):

   getNumColumnsForGroup: function(group) {
      if (group[0].length == 1 && this.eventData[group[0][0].id].room === '') {
        return 1;
      } else {
        return keys(this.roomsCols).length;

and keys(this.roomsCols) return an empy list so that length is 0 resulting in NaNs later on.

If anyone has some hints how to best fix this ( I could fix it by assigning dummy rooms for now, which is not too nice), I would try to write a patch. Note that the backend layout is correct.