Using a rubric to grade in Canvas can be an exercise in frustration. Even the official documentation (How do I use a rubric to grade submissions in SpeedGrader?) starts by having you resize the parts of the screen, which leads to the submission being too small, or the rubric being too small and the grader needing to scroll.
This idea is to have the View Rubric button optionally allow the the rubric to be popped out into a new window that can be used alongside SpeedGrader's other tools while grading.
Alternatively, Canvas already supports in-page "window like" panels that can be moved around, resized, and interacted with while being part of the page you're on (example: Configure External Tool as part of assignment creation). This could be an alternative to a full pop-out window.