I like the little indicator bar showing where the results are. In the final version, will the dark grey bar showing the current position be limited in width to the currently displayed range? One could also think of putting it vertically next to the actual scroll bar and using the slider as position indicator, but that might be difficult with infinite scrolling. Or an additional medium grey region indicating parts that are loaded, but not on the screen right now (kinda YouTube-style)?
If one scrolls down to a search result far down on a long board, a lot of topics have to be loaded. Maybe there should be a way to load and show only topics with hits?
I can't think of any cases where your concept wouldn't work right now. What will surely come up at some time is searching for wikimarkup used when composing a post. This might be quite expensive if the markup is not in the database, but that's not really the topic here.
Best wishes
Hhhippo