This shouldn't be a constant.  MessageListCard._hideSearchBoxByScrolling is our logic that dynamically scrolls to avoid hardcoding like this; it can save the value off and/or otherwise relay it to the MessageListTopbar.  (At some point we will fix that method to avoid the reflow it compels, but then if we do a horrible constant there, we ideally will just fix it in that one place, or at least it'll be obvious we have to fix this place too.)