One needs to understand context here. Title Case makes sense for short, snappy commands. It is imperative. It is clear. It’s exactly the opposite of what I see at the bottom of this comment field, where it says ‘Go full screen’. What the heck does that mean? Like, Go Full Monty? What’s more, it’s grey in colour, suggesting that it’s disabled.
While it’s helpful that hovering over it produces a slightly confusing explanation ‘Every response is its own story. Expand to the full-screen editor at any time.’ I think I understand that it means that ‘When you click on this button, you can edit in a full-screen editor, where each response looks like a full story. This full-screen editor is always available when you need it.’ However, hover text shouldn’t be a band-aid to fix problems with the initial wording of a button, especially because it’s not available in a number of instances (most notably, for tablet and phone users).