Having finally gotten round to uploading the dread
tikzmark package, I now approach the next task which is linking to the original question here and putting the "This question led to a new package" banner on it.
Only I'm not sure which to designate as the true origin.
How can I get an underbrace and an overbrace to partially overlap in an equation? The actual first appearance. However, its use is a bit overkill for the question and it can't really be said that the question led to the package.
Adding a large brace next to a body of text This is the one where others started noticing it and is the one that many link back to when referring to it. But still, the tikzmark itself is not about putting brackets next to text.
tikzmark to have different behaviour if first run (and mark locations not yet available) This led to a major revision, including the introduction of special coordinates. Moreover, the question was actually about the tikzmark.