Is this rear der. rubbing normal?

I started fiddling with my bike again (which is not a good idea for me) and I noticed that when I'm in the small/small combo (34x12), the chain rubs on the rear derailleur. Not that I would use that gear much, but I don't know if it has always been that way, or if I messed something up. The only thing I touched on the RD was the barrel adjuster.

In the pic, the bike is upside down, but even when it's right-side-up it still rubs a bit.

The barrel adjuster mainly moves the RD sideways. If it causes any motion in the vertical at all I'd consider it marginal in terms of your issue.
Even if your bike is all stock it's quite common that chain length has been chosen rather arbitrarily. Put the bike in big-big and see what chain tension and RD angle you get then. Maybe you can afford to lose a few links?
If your bike isn't all stock, or was assembled out of spec. anyhow you might have an issue with the RD chain wrap capacity. Depending on the span between biggest no of teeth and smallest no of teeth you need different cage lengths in the RDs to cope with the (sometimes) excess chain.

Put the chain in the big-big combo. If the chain can easily do this, it's possible to move the rear derailleur cage forward even more, then you need to take 1 link out of your chain.

50/34 compact? even with a 12-27, take a link out.

Thanks for the help. I took a link out, and now I have about a half inch of clearance in small/small. Shifting works in all 20 gears (50/34, 12/27), so the chain musn't be too short. If that were even possible???

Granted, I have not put this shorter chain under extreme load yet. Couple years ago I had chain slippage in my 105s when I replaced the chain. That was frightening.

It is possible to have a chain too short. If you cannot shift into your lowest gear or your highest gear, then the chain is too short. I even like to make sure that I have enough chain length to shift to my largest cog and largest chainwheel. Even though you normally would not ride in that gear combination, I want it in case I accidentally shift into it.

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Sounds like you've corrected the problem. I've found that slippage under load happens when matching a new chain with a worn cassette. I need to change cassettes on every second chain replacement, after about 8-9K miles.

Concerning chain length, another check I've used is that with a proper length chain, the RD cage pulley axles will align vertically when in top gear (eg, 53/12).