The native Touch ID prompt currently appears before the corresponding Chrome UI sheet. This is because we request the UI sheet do be changed and then immediately dispatch the request rather than waiting for the UI sheet to get painted, animated and whatever else happens invisibly in the background.

crrev.com/c/1185237 addresses this by adding a fixed delay to the request dispatch. We want to have small delay anyway in between the Chrome and native UI appearing so that the two don't render at the same instance, which might be confusing. However, I fear that this fixed delay might not be good enough because the time it takes for the Chrome UI sheet to render might be too dependent on hardware, system load etc. Therefore it would be better to start counting the delay **after** the UI sheet has fully animated, but I currently have no idea if there is a corresponding observer/event that we can watch for to achieve this.