So I tried out the new reader again today (I've been using Legacy for a while now) and it took me a while to puzzle out what it was about the lack of unclickable whitespace around a page that bothered me so much.
Basically, I switch between windows a lot (sometimes I have multiple windows open side by side), so I usually click somewhere in the whitespace around the page to bring focus back to the browser window. And because of this ingrained habit, when using the new reader, I kept accidentally hitting the huge click area, which caused the next page to load, and I kept going, "aaaaaaaa, no, don't do that, I'm not done reading that yet!"
The sidebar or middle click were suggested as alternatives, but the sidebar is just so much tinier area-wise, and it's just slower clicking there (Fitts' law). And if I middle click, I have to click again to get rid of the multidirectional scroll cursor thingy that pops up. This all just contributes to making the reader feel more constrained and stressful.
It's great that there's an option to configure this, though: First, enable
Display advanced settings, and then select
Input settings > Tap/click target area > Images only. Phew, much better!
Though it'd be nice if I could set a light site theme where the container background is slightly darker, so that it's more obvious which areas are safe to click (the dark site themes don't have this problem). e.g. something like this: