Note: Demo recording is of limited value as it depends not just on using the exact same version of the game, but even the compile options and platform can matter. Use of video recordings should be the prefered way to report issues.
When you find a bug which has steps that are difficult to explain, you can save the development team lots of time trying to repeat your bug by recording your gameplay.
When you record your gameplay, you record the keypresses you made during gameplay into a text file. This can then be run by someone who needs more information about your bug.
You need to be able to use the command line. Open the command line, and run SuperTux with these arguments
supertux2 <level file> --record-demo <demo_filename>.txt
<demo_filename> should be a path on your computer where you wish to save the demo recording.
Note: You have no obligation to use the extension .txt, but this means you can easily just drag it into the GitHub comment box
Then, after recording your bug in action, test it out using
supertux2 <level file> --play-demo <demo filename>.txt
Finally, drag the demo file into your issue report
Also drag in the level used to record the gameplay (rename to
issueXYZ.stl.txt as GitHub is okay with .txt),
or name it if it exists in an addon or is included already in the game.