Please see notes at The WebKit Open Source Project | WebKit i.e.
WebKit is an engine, not a browser.
The scrollbar problem is related to Suse Linux, Webkit and the CSS used by Manager. You did not indicate with which version of Manager you did not have this problem and at which version it started occurring. I suspect it may have to do with a main css change in post 57 of the following discussion:
lubos May '21
I appreciate the feedback and I know all of you want to make Manager better. I accept the criticism that if business has a lot of tabs, the constant scrolling is unbearable. So I came up with the following improvement in the latest version (21.4.64)
As such it may be useful if you could install a Manager version older than 21.4.46 from https://github.com/Manager-io/Manager.AppImage/releases. Beware that it may be important to temporary move the data folder to another location so the older version won’t access them. You have to do that before the install of that older version. If it works with the older version than @Lubos may need to revise the CSS to be compliant with webkit and render the scrollbar as expected.