UI should use maximum browser screen real estate when it can

[thadguidry]

Maximum browser screen real estate is not taken advantage of, this results in Original and Changed areas to be less readable since the viewport on each is greatly reduced and word wrapping is often, instead of on occasion, even for those of us with very large portrait monitors.

Both box-border (box-sizing) and flexbox can use sizing attributes that use a percentage of available area.

div.diff-card could then use this available space to elongate both in height and width on standard 1920x1080 or larger screens.