Hi Chris,
It's been year, so you have likely sorted this out by now, but I noticed last Fall, maybe Spring that the interface had gotten really buggy with any writing device and browser, but I've got an iPad Pro that solved my issue. However, our instructional assistants are struggling to help our online students with feedback on their graded math papers. So I've been searching around for a similar suggestion for how to make Canvas work in the browser based speed grader. It looks like this is classified as known issue that the Canvas Engineers are still working on.
I love my iPad, but they are too expensive to outfit out instructional assistants with in the classroom. Hoping this issue gets resolved soon. If any Canvas moderators are out there, would love an update on the timeline for resolving this interface issue.
-Jennie