Came to report these same two issues. The second one seems like a simple bug; I'd expect the text column to be centered in all cases. Not sure what the solution is for the first problem. Even in a normal width window the text box feels too narrow. Multi-column layout is too hard.
There's also a small padding issue[1] next to the sidebar when the column width is set high.
Otherwise it looks pretty good.
[0]: https://jagger.co/downloads/modern-wiki-wide.png [1]: https://jagger.co/downloads/modern-wiki-padding.png