keyboard scrolling spring damping animation doesn't finish and leads …

…to endless flickering scrolling

https://bugs.webkit.org/show_bug.cgi?id=229697

Reviewed by Tim Horton.

The damped oscillation animation of the smooth keyboard scrolling
went into divergent vibration if the frameDuration isn't short
enough. It happened with debug builds with enabling debug
loggings.

Stop the spring if it reaches the ideal position.

* Source/WebCore/platform/KeyboardScrollingAnimator.cpp:
(WebCore::KeyboardScrollingAnimator::updateKeyboardScrollPosition):

Canonical link: https://commits.webkit.org/255306@main

Source/WebCore/platform/KeyboardScrollingAnimator.cpp

@@ -218,6 +218,13 @@ void KeyboardScrollingAnimator::updateKeyboardScrollPosition(MonotonicTime curre
 
     m_scrollAnimator.scrollToPositionWithoutAnimation(newPosition);
 
+    // Stop the spring if it reaches the ideal position.
+    FloatSize newDisplacement = newPosition - idealPosition;
+    if (axesToApplySpring.width() && displacement.width() * newDisplacement.width() < 0)
+        m_velocity.setWidth(0);
+    if (axesToApplySpring.height() && displacement.height() * newDisplacement.height() < 0)
+        m_velocity.setHeight(0);
+
     if (!m_scrollTriggeringKeyIsPressed && m_velocity.diagonalLengthSquared() < 1) {
         m_scrollController.didStopKeyboardScrolling();
         m_velocity = { };