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

[thexeos]

Resolves #17206

What kind of change does this PR introduce?

• Bugfix

Does this PR introduce a breaking change?

• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• 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]

UI Tests Results

    1 files     97 suites   37s ⏱️
1 019 tests 1 019 ✅ 0 💤 0 ❌
1 038 runs  1 038 ✅ 0 💤 0 ❌

Results for commit f764d29.

♻️ This comment has been updated with latest results.

[rstoenescu]

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