When I press ll in Magit's status buffer, then it shows a log buffer listing commits. Then I want to show details about the commit at point in another window, but at the same time I want to stay in the log buffer/window. If I press RET, then it shows another buffer with details about the commit, but it also jumps into the new buffer. How to keep the log buffer selected?
Instead of RET use SPC.
The first time you do that it will show the commit in a new buffer and display that buffer in another window, but it does not select that window/buffer.
Pressing SPC again will scroll in the window which displays the commit buffer, so you don't even have to go to that buffer if some of its content is initially hidden because the window is to small.