It should be easier to change your scrollbar back from the ridiculous default, where it disappears into a tiny vertical slice that only reappears when you hover over the thin vertical slice (!) which is such bad UI design, it's probably intentional. Normal people have clamored for years to get rid of this and go back to the legacy thick scrollbar design. I prefer a nice fat, visible, scrollbar because, while writing in a very long document, which I do often, I can glance at it and instantly know where I am in the document -- but try and do that with a scrollbar so thin you have have to hover over it to find it. Go ahead, I'll wait.
Not instantaneous, is it? This design makes my use case entirely useless. As a software developer, I reasonably despise the extra milliseconds I am forced to use for repetitive tasks, like having to use the mouse in order to make something visible which used to be visible with a single millisecond-glance.
And to make matters worse, alas, take a look at what it takes to fix: The accepted answer at AskUbuntu is so long you literally have to scroll it. Same with the next two answers. Buried further down the page, as Step One of multiple steps, in the fourth answer, is the simple truth which will set you free.
This is how I fixed it in System76's PopOS 21.10, which is basically a well-done flavor of Ubuntu.
Make a backup of /etc/environment. Run "sudo nano /etc/environment" and add the next line: GTK_OVERLAY_SCROLLING=0
That's it. The rest of the absurdity on this topic you can ignore.
Oh, and you'll need to logout/login to reload the environment after you make this change.