Thank you for coming to the Community! I can understand why making sure your contacts can see the whole message is important. However, there is not currently a way to record in our reporting whether the recipient has displayed the email in its entirety. The system records that the email was opened based on two criterias: 1) the invisible image we embed in the email is triggered when images either automatically or manually display, or 2) when someone clicks on a link in the email. This article explains it in a bit more depth.
I will make sure to pass your inquiry to our developers as feedback.