Flexbox 4 unity - in-game resizing UI
Live layout - players of the game/app can drag UI elements to resize, FlexBuilder handles all the dynamic relayout (including adding/removing scrollbars, reizing UI, auto-margins between rows/cols) automatically.
Images are now auto-sized and aspect-ratios auto-maintained correctly, even for radically different size/shape images.
These features will be in the FlexBuilder v4.6 / v5.0 releases.