Yes - Geant4 (from CERN) and OpenFOAM for example.

I'd suppose Linux device drivers count, as does Yocto recipes and one of the main init systems (runit)...

I once wanted to modify the sources of the XED text editor because I wanted to disable 'search as you type'. It makes searching very large files impossible because as soon as you type the first character of the search string, the editor begins to mark every instance of that character. This takes ages (sometimes more than a few minutes) and while this is ongoing, it is not possible to enter any additional characters.

But to my surprise I couldn't find anything in the source code to change this. I guess it is a default of the GTK framework used to build the editor.

Yes, I maintain a few patches for FLOSS programs that are unmaintained, but still work fine after patching.
Come to think of it, I also have 1 or 2 for programs that are maintained because I don't like part of the functionality.

I had to do some hacking to make this code build. I didn't actually modify the program itself, so much as hack the make file to make it actually detect the third-party *.h files it needed to finally produce a build. IDK if that counts, since I didn't actually change the C code, just added some -I arguments to the compiler.

Hacking the build system defiantly counts. Without working build systems you'd have no FLOSS programs -- or at least none that could be built with any regularity.

Yes .. but it is an Android Application that my script automatically compiles when the upstream source changes ...

They did not ask if it was for Android, just if it was open source.
Granted, I would have used FLOSS in the question, had I worded it but I didn't write it.

Yes. Notable examples include:

a.) One of the challenges of upstream packaged products is that they come with a built-in selection of options for the software in question. That works well for most people most of the time, but doesn't work well if you need a non-standard set of attributes.

b.) Less often to accomodate builds with non-Linux compilers (esp. those not freely available or for non-Intel CPUs) ... there are variations (sometimes subtle, sometimes not subtle) in compiler flags if you're not using GCC.)

thx

Back when the Freevo DVR project was still active. I hacked the record command, so it would record from a Hauppauge HD PVR. It worked very well for several years thereafter.

It's a very different kind of business. If you are the sole programmer of a program you (should) understand everything and know where to go whe fixing a bug or adding a feature. On the other hand, if you contribute to a project that other people have worked on, you need to figure out things before ever trying to modify the source, which is more time consuming and involves asking questions on the mailing lists, reading/searching for earlier bug reports, understanding how the code evolved to its current state by reading the commit logs and blames...

Yes modified a whole lot of them, but previously; hex edited various binaries, manually modified boot sectors, repacked BIOS images, rewrote some very small assembly stuff, etc.
Must say that after all those low level adventures, reading and editing well documented and clean C/C++ projects has been much more enjoyable experience.
I suppose systems were always more interesting to me so I never really bothered with application coding, especially not java.

What kind of edits for example ?

You know some old executables are hardcoded to load unavailable resources, like servers which aren't there anymore?
So the kind of edits that make unconfigurable things configurable, I guess. Why?

"Asking for a friend" would be a nice excuse for my ignorance. I have never had the idea to do that, but it's good to know in case I get stucked too. I wonder if I could hexedit those old flash games I downloaded which require to be run from their original website