Indeed, sometimes I'm stuck "watching" something on the screen (it looks like I'm watching a movie) maybe for extended periods of time. Then suddenly I jump and grab a pen and paper, and in a couple of minutes I get everything I've been building up written down, which helps me in solving the last details.
The opposite is true too, sometimes I'm in a bind and can't get myself to work on some task trying to get into flow, and it turns out fiddling like that (opening random terminals, typing useless commands, navigating some code and skimming through some source file) definitely looks like I'm frantically working.
also, browser compatibility, that also requires load of prep work because of the layers of workaround to get stuff working right.
Worth noting, this process can be significantly sped up if you have a set of peers (or smarter) people to discuss with.