This might only be useful to me and a potentially a few other people and requires the Horizon theme be enabled. It has a viewing mode called “Peek” that moves the post compose window into a vertical column on the right. This is extremely useful if you’re multi-quoting in an extremely tall message, or scrolling to grab quotes from multiple topics or messages.
You can essentially browse the entire forum side by side with your message while composing to locate quotes. This can be done in non-peek mode as usual, but I always find the short compose window at the bottom sometimes difficult to keep track of even though you can stretch the compose window up.
I’m unsure if any other themes have this function, but this is a Discourse-created default theme which is why it probably does fancy tricks! I hope other theme creators incorporate this.