Fix dragging in color area and sync value

[bjarnef]

Description

Fixed bug when dragging handle in color area, where handle jumped to left position reported in #463

chrome_mfUAuZoS74.mp4

With changes in this PR:

chrome_SH1Crfdg5u.mp4

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Motivation and context

How to test?

Screenshots (if appropriate)

Checklist

• If my change requires a change to the documentation, I have updated the documentation in this pull request.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[github-actions]

Hi there @bjarnef, thank you for this contribution! 👍

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• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
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[bjarnef]

@iOvergaard could you have a look at this one? :)