Show all forms of distributive law as popups.
Show popups when hovering subexpressions of the applicable expression.
Move the step checkbox out of the stepNumArea for better-looking highlighting.
Make the rule selector options always visible, not hidden in a dropdown list.
Call selectionChanged directly in the selection management primitives.
Rename rule selector refresh to update.
Make a single element for each step, defining the area sensitive to step hovering and selection.