It's been around since 2015, so pretty much from the start: https://signal.org/blog/signal-desktop/

Any idea about how the security posture changes? I would expect some significant tradeoffs are required to make all your messages appear in your browser

The Chrome app has been deprecated for some time - the current desktop version is independent of browser and available for Windows/macOS/Linux/iOS: https://signal.org/blog/standalone-signal-desktop/

I haven't used it in a while but I think it's probably similar to WhatsApp where it basically just tunnels it through your phone without actually running a Signal client on your machine.

It's not, once linked the desktop client can be used independently of your phone.

i'm not 100% sure how it works, but it seems to sync some messages. Not working perfectly as my desktop client does not have the full history from a chat, even though my phone does. It only has some of the messages from where the desktop client was first started.

It's quite odd to brand a web app as a "desktop" app, but it does fit with Moxie's unfortunate decision of trusting Google et al.

The Chrome app has been deprecated in favour of a true desktop app for several years: https://signal.org/blog/standalone-signal-desktop/

A true desktop app in so much as it bundles a standalone browser and uses that to render the web app part...