Hello, tom2148.
Let's start off by checking the helpful steps in this article: Allow or block pop-ups in Safari on Mac. If these are the steps you are following, then let's look into the Mac settings to see what might be affecting this.
To start off, we advise that you restart your Mac (Shut down or restart your Mac) and make sure it's fully updated (Update macOS on Mac). Once those are done, please test this issue.
If the issue persists, continue by testing to see when this behavior occurs. First, put your Mac into safe mode and test. Booting into safe mode will disable any startup and login items from automatically opening, which can help us to isolate the source of the issue further. When you do this, you’ll see some unusual behavior from the display; this is to be expected. Please follow the steps in this article: How to use safe mode on your Mac.
Next, let's see if this behavior persists in a second user account on your Mac. Please follow these steps to make a test account that can be deleted later: Set up users, guests, and groups on Mac.
Let us know how this goes, please. Thanks and have a fabulous day!
