Diff Navigator for GitHub by Dean Attali

GitHub Diff Navigator allows you to easily navigate through the changes in a file that has been edited on GitHub.
Code available at: https://github.com/daattali/github-diff-navigator-extension/

Description
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This is a Firefox extension that allows you to easily navigate through the changes in a file that has been edited on GitHub.

You may have noticed that after editing a file through GitHub and previewing your changes, there is no easy way to navigate through the changes in a nice "next/previous change" manner. Most file formats only show the lines that were changed and several surrounding lines, so it's usually not a terrible inconvenience, but for any markdown document (.md) the situation is different. GitHub shows the entire markdown document when previewing the changes made, which means that if you're making a few tiny edits to a large document, it's very hard to find those edits and see what's changed. Rmarkdown (.Rmd) documents behave similarly, and potentially other markdown-type documents as well.

Enter: GitHub Diff Navigator.


Motivation
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The idea for this extension came when I was reading through Hadley Wickham's Advanced R book. Every time I found a few typos in a chapter (which is a single markdown document), I would edit it on GitHub and make a pull request.
To make sure I didn't make an idiot out of myself, I wanted to double-check my edits in the Preview tab, but I noticed the problem described above. I didn't trust my eyes in finding the few edits in such large files, and I ended up using the DOM + JavaScript console to help me find where the edits were.
Then I decided that playing in the JavaScript console wasn't hardcore enough and I decided to spend the next two days making an extension that will help me find such changes quickly in the future.


Features
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- GitHub Diff Navigator is non-intrusive and will only show up when looking at the Preview of edits made on a file using GitHub.
- GitHub Diff Navigator will not take up any resources or show up in the Task Manager when you don't have a valid GitHub edited page open.
- Works with multiple GitHub tabs that are open simultaneously (every time you switch tabs, the extension will update to show you the edits in that tab).
- Automatically detect whether the file being edited is markdown-type or not, and seamlessly works for both.
- Clean and intuitive UI with helpful messages shown when no edits are found.