How does changing the border color do that exactly?
But there is still a lot in the UI surrounding the composer that is not the composer itself and can’t be clicked, and now it’s harder to see where the surrounding part ends and where the composer begins.
Honestly, now if feels like there are just a whole bunch of floating elements (i.e. the composer and all the buttons above it) with no border at all because the border lacks contrast so much. Even the separators between (for example) the italics button and the link button are basically pointless now because it’s nearly impossible to see them.
Yeah, clearly I’m missing the point of the change, but I definitely get having an acclimation period. I’m just finding this change particularly jarring for reasons that are hard to explain.
Looking through the dev console, I see the border went from --primary-medium (#909090) to --primary-low (#313131). which is a pretty drastic change. I just tried it with #616161 to split the difference as you mentioned, and it really is a huge improvement. It provides more much-needed contrast, while not taking so much attention like the original #909090 did. Experimenting with various values, I think even something like #515151 would strike a really nice balance between contrast and not being overwhelming. Hopefully this is something you’ll consider after a little bit of time.
(I noticed there is a --primary-low-mid all the way up at #7a7a7a. Guess it’s time to add --primary-low-not-quite-mid for #515151. 
)