Replace site title & logo with "< parent breadcrumb" after scrolling down on mobile

[psychobunny]

Right now, the topic title is totally useless because it only shows about 6 characters before being chopped off on a portrait Nexus 5 (much worse on smaller devices!)

"< parent breadcrumb" should probably be truncated to no more than 25% of the max width I imagine.

[julianlam]

👍 I like this idea 😄