When reviewing a pull request's diff, if one file is changed extensively, I often have to scroll through quite a few lines while reviewing the changes to that file. Sometimes I forget what file I'm looking at by the time I get to the end, and I have to scroll back up to the top of the file to see the header indicating the file path. This precision scroll jump can be difficult if there are lots of files in the diff; if you go too quickly, you can miss the separation padding between files. Then I have to jump back down to the next file to continue reviewing.
I suggest floating the file header with the file path so that it is always visible, or "sticky" to the top of the screen. When you begin scrolling through a diff, eventually the diff of first file listed will begin to move offscreen. When this happens, the filename header should remain onscreen by sliding down from its original position. Only when the file diff is completely scrolled offscreen should the filename also scroll offscreen.