It’s a Discourse thing and not something that’s easily changed. It’s been discussed dozens of times over the years upstream. It’s not gonna change.
As a crude workaround: You can get the raw (markdown-formatted) full text of any topic with the url https://ziggit.dev/raw/<topic id>
e.g. https://ziggit.dev/raw/15710 for the topic you linked. Your browser’s search works there then.