It may not be easy to trigger this problem in standard Moodle. We found it using https://moodle.org/plugins/quizaccess_honestycheck, with quite a long statement that students are required to agree to.

Steps to reproduce:

1. Create a quiz, turning on a number of options that put content in the start attempt dialogue. E.g. require password, and the honestycheck plugin mentioned above.
2. Try to start an attempt at that quiz in a mobile browser.

Expected result: The dialogue contents is scrollable.

Acutal result: You can't scroll, so you can't see all the content, and you can't get at the buttons you need to proceed.

The problem here is that basically I stuffed up in MDL-46091. I added too much CSS in mod/quiz/styles.css in an attempt to make the dialogue look pretty. Unfortunately I introduced this bug in the process.