The Practise/Quiz/Challenge dialog would be better if it took 100% of the screen, especially on mobile/tablet devices, simply because if you accidentally touch/click outside the question area it closes down the dialog.

My kids do this all the time!

It's great that you can simply resume where you left off, but you still lose any workings out/notes etc.

You can obviously use the large X to close the dialog.

Thanks,

Rich.