fix(QScrollArea): correctly compute snap position for thumb #17206

[thexeos]

Resolves #17206

What kind of change does this PR introduce?

• [X] Bugfix

Does this PR introduce a breaking change?

• [X] No

The PR fulfills these requirements:

• [X] It's submitted to the dev branch (or v[X] branch)
• [X] When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• [ ] It's been tested on a Cordova (iOS, Android) app
• [ ] It's been tested on an Electron app
• [ ] Any necessary documentation has been added or updated in the docs or explained in the PR's description.

[github-actions[bot]]

UI Tests Results

    1 files     97 suites   37s :stopwatch: 1 019 tests 1 019 :white_check_mark: 0 :zzz: 0 :x: 1 038 runs  1 038 :white_check_mark: 0 :zzz: 0 :x:

Results for commit f764d29a.

:recycle: This comment has been updated with latest results.

[rstoenescu]

Thanks for contributing! This will go into Quasar v2.16.12