fix: scrollstart/scrollend custom events raised twice in fast-horizontal-scroll (and fluent-horizontal-scroll) navigation

[vnbaaij]

🐛 Bug Report

When using the fluent-horizontal-scroll, each click on a flipper raises two scrollstart and two scrollend custom events. Both scrollstarts and both scrollends contain the same data.

💻 Repro or Code Sample

Can easily be reproduced by putting a breakpoint in the web-components.js file

🤔 Expected Behavior

Only one pair of scrollstart/scrollend events should be raised

😯 Current Behavior

See bug report

💁 Possible Solution

🔦 Context

🌍 Your Environment

• OS & Device: [e.g. MacOS, iOS, Windows, Linux] on [iPhone 7, PC] Windows
• Browser [e.g. Microsoft Edge, Google Chrome, Apple Safari, Mozilla FireFox] Edge Canary
• Version [e.g. 1.8.0]