Clicking on an image thumbnail opens a full page with no way to close it other than the back button.
Have copied the product-template from another website where behaviour is as expected (Modal), but has made no difference.
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If you don't want to share the storefront password (NOT the Admin password) then take a look at the browser debug console logs. You might find some errors showing there that give you some clues into what might be going wrong.
This is an accepted solution.
If you found the inspect tool, you're close. The debug tools is found in most browsers and will fall under developer tool menu (or something like it).
There's a couple possible of reasons here but let's start with the first one. You have looked to have added JotForm into your shop (or some app has) and it's loading another version of jQuery. Multiple versions can be ok, but not if they are loading incorrectly. Looks like this is the case here so some connections to the first version is lost as this one loads.
See what happens when you don't have that code load.
If you're only getting started with JS debugging in a browser, here's a link that you might find interesting: