servo: Merge #14597 - FIX for Implement scroll transactions (from gterzian:fix_scroll_transactions); r=mbrubeck

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Follow up on https://github.com/servo/servo/pull/14470

@mbrubeck @KiChjang @glennw I just found out in the context of https://github.com/servo/webrender/pull/600 I forgot to add a case for the very first scroll event, or else the scrolling on that PR, in a non-Mac OS environment, will only start after an 80ms pause following the initial scroll event...

Sorry this slipped through my initial testing...

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Source-Repo: https://github.com/servo/servo
Source-Revision: 139c111091210a5002bda668d14debb2e1c68ca9

servo/components/compositing/compositor.rs

@@ -1081,6 +1081,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                               delta: TypedPoint2D<f32, DevicePixel>,
                               cursor: TypedPoint2D<i32, DevicePixel>) {
         let event_phase = match (self.scroll_in_progress, self.in_scroll_transaction) {
+            (false, None) => ScrollEventPhase::Start,
             (false, Some(last_scroll)) if last_scroll.elapsed() > Duration::from_millis(80) =>
                 ScrollEventPhase::Start,
             (_, _) => ScrollEventPhase::Move(self.scroll_in_progress),