I had a bit of a shock while looking over a section of my book on typesetting special characters to discover that my example of how to typeset a four-dot ellipsis in LATEX was coming out like:
when I was pretty sure it was supposed to look like:
It was pretty easy to discover that this was because in XeLATEX (and LuaLATEX), \ldots was no longer typesetting spaced dots but using the Unicode … character instead. The appearance, as evidenced above is quite different and the Unicode character doesn’t compose nicely with a following period (as required by some styles, most notably MLA, but some others as well—the example made it into the book originally because it was repeatedly asked about when I was teaching LaTeX classes back in the 80s/90s).
With some digging into source2e.pdf, I turned up \UndeclareTextCommand which lets me undo this. It’s just a matter of adding
\UndeclareTextCommand{\textellipsis}{TU}
to my document class to get the intended result and my ellipses are returned to normal. Unfortunately, I’ll also need to include a mention of this into a sidebar for those who might encounter this edge case on their own.
Update: While writing the text for the book, I discovered that the sequence …. came out almost correct in Monotype Ehrhardt (the spacing is still a little uneven, but not as dramatically so as with Latin Modern), which made it a little complicated to write the sidebar. I had to switch to Latin Modern for those ellipses to illustrate the problem. It looks fine on the blog too. Latin Modern’s ellipsis is broken!