One feature I look for in any webview is the ability to make parts of the UI transparent while still allowing mouse events over transparent pixels to seamlessly fall through to the window below while those events (not just clicking but hovering) over non-transparent pixels be intercepted as usual.

IMO the old ActiceX IE browser control had an exceptionally good (most performant and natural) implementation of this feature. Unfortunately replicating it in chromium based webviews appears to be a lot trickier.