Great report, thanks @Jagster. This bit was very helpful:
When you perform a touch action on an element that doesn’t scroll, the browser (Safari in this case) propagates that event to the parent elements. In the composer, that means it propagates it back up to the body and then scrolls that body. Even though a) the body isn’t visible and b) the editor is in a fixed-positioned element, which usually doesn’t warrant scrolling the body.
So, to fix for this issue, we added some logic in JS that stops the touchmove event from doing its thing while the textarea is short. But inadvertently, that causes issues with text selection.
Just merged this PR to fix: FIX: Allow `touchmove` in composer when there is a selection by pmusaraj · Pull Request #30957 · discourse/discourse · GitHub