In regards to endless scrolling, it is currently difficult to implement right now
The only issue with this is that Sveltekit's client router doesn't preserve the app state when clicking the back button, meaning you'll lose your scroll position. I'm unsure how I'll fix this without changing the way data is fetched and how the layout works, but I do plan to overhaul the app to be like an SPA soon.
So, it is a known feature request, but not a priority ATM.
In regards to resizing the side bars, you can currently resize the left/community bar. In the bottom right corner, there should be a triangle that you can drag to resize the sidebar (within certain limits). The size will be remembered for the current session, but if you refresh, it will be lost (remembering this setting is another feature request being considered).