you might be able to replace the outer element with the inner one, and put the "chat" text in the content of a pseudo-element, but it's getting late for me so I'm about to head off.
(also please ignore my abuse of position absolute, I'm trying to kick the habit, and you could probably use grid instead, but this was quicker for me right now and not the focus of the question)
a before with a rotation of 45 deg, a locked width and height of , whatever border-radius you want on all corners, a solid border of x pixels and a border-color only on the right facing sides for the triangle thingy
an after with a border-radius 100vw, same border as the before, a background color matching the backdrop
the same border than both pseudo Elements and the same background-color as the before on the actual element.