For full disclosure, I should mention that my last mockup changed more than the tabs and border for the open area, in case that matters. Or if anyone cares. Blender always seems to have everything off by a pixel or two. LOL
The chrome for unselected tabs were removed, as mentioned. But the icons were also moved left to be better horizontally centred in their column.
The “change editor type” button was desaturated to match the energy of the unselected tabs, but I also removed the chrome around it too, implying it could become move noticeable on hover.
The entire open area was moved up slightly so that the padding between the region title, “Relations”, and its content is smaller than the padding between the bottom of the open area and the following title.
The contents of the open area were moved to the right so that the right margin is smaller and the property decorator icon is horizontally centered in it.
The “open area” arrows are moved up slightly to better align with the titles. The “down arrow” is moved down and to the left a bit so that it rotates in a more plausible way.
The “drag section” gripper icons are raised a bit to better align with titles.
Here is basically that same image again, but with smaller border on both sides of the open area:
