Clicking a link in email opens a vertical window to the link, which is unreadable and VERY annoying

You need to explore multitasking - as you have a slide-over view of Safari enabled.

Key to accessing and controlling multi-tasking is the small “three-dot” button that is found at the top-centre edge of each App Window. Tap the multitasking button at the top of the slide-over window to expose a menu - from which you can change the window view.

More information about using multi-tasking can be found in this support page:

Use multitasking on your iPad - Apple Support

The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:

iPad User Guide - Apple Support

https://books.apple.com/book/id1567104892

You are unlikely to have encountered this view due to iPad settings - but have (perhaps inadvertently) selected a slide-over view when using Safari...

When using Safari, this multitasking button is very close to the URL/Address bar; combined with a likely “conditioned habit” of tapping the centre of the Safari Address Bar, this is a common trigger of unexpected split or slide-over windows.

Be aware that text shown in the URL/Address field is centre-justified; you can tap anywhere in this field to edit the website address. If you follow the usual usage pattern, you likely have a “conditioned habit” of tapping the centre of the field. Instead of tapping the centre of the URL/Address bar, try to consciously tap either side of centre; in so doing, you can completely avoid accidental activation of the “three dot” multitasking button.

Over a short period of time, you will subconsciously develop a new conditioned habit when using Safari - and will hopefully avoid the common cause of accidental triggering of multitasking views.