I'm not sure how much control bookdown has over the Git pages styling, but wanted to file just in case. To reproduce:

1. Open a Git book (e.g. https://bookdown.org/yihui/bookdown),
2. Initiate a search (click the magnifying glass),
3. Press the spacebar (e.g. as though you were searching for a single space),
4. Click the main page (unfocus the search dialog).

The left sidebar is now empty without any real indication of why.

Note that since Gitbooks seems to remember the last search term used, if you refresh the page you should then see the same UI, making it a bit more confusing in that case.

It seems like the search dialog here should get a bit of minimal styling (e.g. an magnifying glass on the right + a background)?