Please stop putting exercises and quizzes in pop-up windows. The current design is very frustrating for my learner for the following reasons:
1) The "floating" nature of the window means that the slightest stray touch outside the pop-up causes the exercise window to disappear.

2) The floating window does not make use of the available screen real estate. As a result, more scrolling is required that is necessary.

3) The video content suffers from the same design issue. It would be best for video of opened in a new tab separate from the main lesson content tab

I love your content and Khan academy has been very helpful for my learner but the current design is a major obstacle to forward movement with the lessons.