This is HA in a browser on desktop
When I have docked windows, I can’t reach the scrollbar as it’s within the docking popup
When using a tablet and pen you can’t scroll at all. You MUST have a mouse with a wheel.
I find this a shocking accessibility issue.
While I don’t have a tablet with a pen, I’ve just tried to replicate this issue with a (windows) tablet and finger, but can’t.
Garbing the scroll bar was kinda hard, but I was able to scroll by just swiping up or down in the middle of the page.
It should be same with a mouse. Hover on the bar, the divider lights up you can’t scroll.
What OS/Browser are you using?
On FF / Win 11, I can use the scrollbar by dragging it with the mouse, a scroll wheel, or pen input when the window is docked while another is adjacent to the right. I can’t use the scrollbar via touch, but the window scrolls with touch.in the middle of the page.
Using Win10, chrome.
On Win11, There’s a small delay before the middle select bar turns up so if you’re really fast you might be able to do it.
I too use FF/win 11, but just tried Chrome with win 11 and the scroll bar stayed large (which is also a little strange)
I’m running out of ideas, but do you have any custom components for themes/UI enhancements?
If so, its possible one of those are causing the issue??? (Maybe, side note, front end dev isn’t really my thing)
As a work around you could try Microsoft PowerToys. While it wont fix your underlying problem, it will give you more control with how windows dock.
I’m pretty sure my stuff is just std. The scroll bar is different on Win10 and Win11.
I have a win11 machine and it’s wide.
That is indeed the important question!
I don’t use ether of that products but can confirm a normal (“large”) scroll bar with FF / Ubuntu 22.04
Maybe you should open a issue at microsoft and/or google? 
So windows 10 bug?