Viewing resolution

No, not with Photos. Tell Apple what missing features you'd like restored or new features added in Photos via Feedback - Photos - Apple

However, you can set Preview as an editor and then open a photo within the library with Preview.

Set the Preview settings to:

and then use this menu option:

To open the photo in Preview select the thumbnail in Photo, then ctrl-click or right-click it and use the command "Edit with > Preview".

You can also open your Photos Library in the free trial version of PowerPhotos.

When you double-click a thumbnail in PowerPhotos to view it, the small photos will be opened at 100%.

For example:

I am keeping a PowerPhotos window open, when I am working in Photos. It is very convenient as a second browser.

It is a bit disappointing, you are right.

With the previous system version it has not been possible to enlarge photo with a poor resolution to fill the screen, because they became blurry. Many users complained because they wanted to be able to fill the screen or zoom in, even if the photo became blurry.

Apple has changed the behaviour, and now we can again fill the screen even with photos that are having a poor resolution. Only Apple forgot to provide a way to see these photos at 100% - now we are having the opposite problem. I hope they will get it right in the next system version.