Text fields in Safari browser not working

I agree that something appears to be stealing focus from Safari.

Here are some troubleshooting steps to follow, in no particular order:

• If Safari doesn't load a page or webpage items are missing - Apple Support
• If Safari is slow, stops responding, quits unexpectedly, or has other issues - Apple Support
• How to use Internet plug-ins in Safari for Mac - Apple Support
• "Safe Mode": Use safe mode to isolate issues with your Mac - Apple Support

  Determine if the problems exist in Safe Mode. Then, restart normally.

Needless to say, if you are using any non-Apple "anti-virus", "cleaning", or "Internet security" products, anything at all in that broad category of utterly useless garbage, don't. A lot of things won't work if you do.

If you don't get anywhere, you can use the following procedure to determine if the problem is internal or external to your Mac. Causes may involve your router, your ISP, the DNS Server(s) it uses, or certain user-configurable options.

1. Shut down your Mac.
2. Load macOS Recovery by holding ⌘ and r (two fingers) while you start your Mac with a third finger.
3. At the macOS Utilities screen, release those two fingers.
4. Choose Get Help Online.
5. Safari will launch, but it will lack your bookmarks, favorites, history and other preferences.

Determine if the problem persists while using Safari in that mode. After that, restart your Mac normally, and reply with your observations.

(I really wish there was an option to just NOT UPDATE, instead of only being able to select "Remind me tomorrow" -

If all you want to do is stop the macOS upgrade notification nag, then download it. The nag will cease. You can install it later, or never.

Having said that failing to keep macOS (and Safari) updated is seldom justified. Time Machine can restore a snapshotted version of the previously installed system. All you need to do is turn it on. For a more in-depth discussion with additional options to consider, read Upgrading macOS without fear.

You are way behind in your High Sierra updates which would also update your version fo Safari to 11.0.3. See if these changes rectify your Safari issues. Otherwise, its like standing blind-folded at the roulette wheel.

The usual drill is to clear your Safari browser history and cache, then relauch with the shift key depressed, and assess if the problem has mysteriously ended.

Then, there is the possibility that some third-party browser plug-in may be out of date, or contributing to the issues that you describe. Same drill for any installed browser extensions. You may have to disable these one at a time until the problem disappears.

You may also possibly have malware that is causing your browser issues. Download Malwarebytes, and see if running it discovers any malware that it offers to remove. It will operate in trial mode, so no subscription to purchase if you aren't inclined. I don't.

It sounds like Safari is switching focus to another App or the Desktop. I don't understand why you are still on 10.13.1 either but I suppose you have your reasons.

FWIW, 10.13.2 was released Dec 6, 2017, 10.13.3 was released Jan 23. I don't know why you aren't seeing updates. I don't know that it would fix your problem at any rate.