モーダル入力フィールドでトピックのページが一番上にジャンプする

[Jagster (Jakke Lehtonen)]

いいえ、あなただけではありません。これはおそらく次の原因によるものです。

iOS 26 bugs with fixed position elements in Discourse

iOS 26 has been officially released, but we recommend holding off on updating until some Safari issues are addressed. These are visual issues that cause fixed position elements on iPhones and iPads to be misplaced after opening the on-screen keyboard — in Discourse this includes the header, mobile footer, composer, and chat. We’re keeping a close eye on this bug and are attempting to build some workarounds in the meantime, but solutions are fairly limited due to the nature of the problem being …

したがって、26.1 で改善されるはずです。