Quite like this, notes:

• It's come up before that it's important users are aware when they're mirror editing.
  
  If the topbar were on my default, this would solve that issue.

• Mixing Symmetry/Mirror terms is awkward here, painting is symmetry, edit-mode is mirroring one side to another, they're not exactly the same although I'm not sure the difference is important to users?

• It would be good if we could draw these buttons without taking so much horizontal space (like icon buttons).

In D4895#112117, @Campbell Barton (campbellbarton) wrote:

Quite like this, notes:

• It's come up before that it's important users are aware when they're mirror editing.
  
  If the topbar were on my default, this would solve that issue.

Right - but that can also be seen as an advantage. There's no extra clutter unless the tool settings top bar in enabled.

• Mixing Symmetry/Mirror terms is awkward here, painting is symmetry, edit-mode is mirroring one side to another, they're not exactly the same although I'm not sure the difference is important to users?

Yes, I don't know why we use different terms either. What IS the difference? I assume that, sometime later on, we could unify this, and allow for Y and Z mirroring in Edit Mode too?

• It would be good if we could draw these buttons without taking so much horizontal space (like icon buttons).

Yes agreed. The XYZ buttons are quite wide here.

One way to make it take way less space, is if the symmetry stuff could be made to work more like eg snapping or proportional editing, with a main toggle, and then settings inside:

Another way is to just make the current buttons take less space, like so:

Committed rB909c0bd0433222bcdb8661c3ecdf6026150557f0

This doesn't include the symmetry popover changes,
these don't fit well with the current logic, since we want to exclude a single panel from the topbar, it means either looping over panels in Python or manually listing panels, neither are convenient.

The best I can think of is defining the panel twice.

EDIT: committed rB91a292ba40b4008263e117955a8366dfc0d963c4

Note that this is an issue exposed by this change, weight painting has two kinds of mirror which is exposed by this UI change.