Please submit bug reports for all of Cura and CuraEngine to the Cura repository. There will be a template there to fill in. Depending on the type of issue, we will usually ask for the Cura log or a project file.
If a bug report would contain private information, such as a proprietary 3D model, you may also e-mail us. Ask for contact information in the issue.
Bugs related to supporting certain types of printers can usually not be solved by the Cura maintainers, since we don't have access to every 3D printer model in the world either. We have to rely on external contributors to fix this. If it's something simple and obvious, such as a mistake in the start g-code, then we can directly fix it for you, but e.g. issues with USB cable connectivity are impossible for us to debug.
When requesting a feature, please describe clearly what you need and why you think this is valuable to users or what problem it solves.
If you want to propose a change to Cura's source code, please create a pull request in the appropriate repository. Since Cura has multiple repositories that influence it, we've listed the most important ones below:
If your change requires changes on multiple of these repositories, please link them together so that we know to merge & review them together.
The style guide for code contributions to Cura and other Ultimaker projects can be found here.
Some of these repositories will have automated tests running when you create a pull request, indicated by green check marks or red crosses in the Github web page. If you see a red cross, that means that a test has failed. If the test doesn't fail on the Main branch but does fail on your branch, that indicates that you've probably made a mistake and you need to do that. Click on the cross for more details, or run the test locally by running cmake . && ctest --verbose.