Funny you say that Alex, because that is actually what I have built my Custom CSS around is display:flex.
I have altered your sbs_tdIndex display settings to change it to flex, and then have hacked up the rest of it to cooperate. I’m sure you can make it better than what I have since I’ve tried to basically overwrite the core script, but I’m more than happy to share what I have.
Attached you will find screenshots showing the customization’s I’ve made. They are available to you if you’d like them.