From a web UI perspective, is there any research or defined best practices in regards to scrolling within a modal window? Please provide specific links, studies, etc.
I am of the school that a modal should always be constrained (within) the user's viewport and if the content is longer, then the content should scroll within the modal.
However, a colleague leans more towards never scrolling within a modal - that the modal can extend (height) beyond the viewport and the user utilizes the browser scroll for accessing the contents of the modal below the fold.
Here's a wireframe comparison for illustration (in a mobile/handset web context):
I should preface my question with that I think modal's should only be used as intended, for small pieces of information or interaction, however we have instances where this is not adhered to.
possible approach. Distinctly maintains only one scroll bar and may address pros and cons of each approach: https://trello.com/card/complete-trello-api-v1/4d5ea62fd76aa1136000000c/1397