In cases where components don’t exist in a hierarchy (for example, header and main body) I prefer to use our injected appEvents object to signal things like this. In fact, there is already a header:show-topic event that is triggered when the topic is changed while browsing around. So the fix was just to trigger that after the topic was updated:
https://github.com/discourse/discourse/commit/05e99a68cec550eb3b1aa9703bdca462e74303e7
Over time I learned to loathe observers. They still have some uses in our codebase but if there’s a way I can instead say “okay, signal that something changed”, preferably by action handler, I’ll always do that.