yuki hello. Thanks for your feedback!
Do you know if other websites/software usually work with that input system "out of the box" or do they require custom plugins or libraries to handle it ?
In other words, do you think this is a feature that's missing from Flarum, or a native feature of your computer that's broken by something Flarum does ?
If this is the first case (missing feature), then we usually let third-party extensions implement these kind of features. If it's the latter (a problem with core Flarum), an issue can be opened and we will consider fixing it in core.
We do have some javascript running in the post composer to handle things like inputting Markdown lists and emoji autocomplete that could be messing up with the default behavior of the system.
If you have your own forum, can you maybe try with and without the Markdown extension enabled to see if it changes something ?