The macOS version does not currently have the ability to edit the split names in the app. You are going to have to open the split file in a text editor and change them there.
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In-app creation is not implemented. Right now you have to manually edit the text file (I believe there are examples included.). Let me know if you have any other questions. I am trying to get back to addressing issues with this app right now.
They are not implemented. It is on the top of my to do list, I just have not gotten to it.
(The reason mostly being that I don't play PC games, so having the timer on top is just the natural way of play for me.)
But rest assured that they are planned. It's just a matter of finding the time.
The way the core of TST is made does not allow for this. It builds based upon what the split file is like (along with other settings not currently in the Mac OS version but intended to come).
Although this could be a feature, generally I use OBS settings to fit it as I need it and I know that TST will always be the same window size. Is there a reason that you cannot just use the properties in OBS (or X-Split) to get the desired results?