use a separate `scrollLocked` var instead of abusing the scrollView's properties to store the locked state;
fixes an issue where the scroll indicators were no longer visible because lockScrollView() was executed twice before unlockScrollView() was called (due to the user changing `showsVerticalScrollIndicator` mid-animation);
The error detail is here:
> Extensions.swift:11:18: error: cannot convert value of type 'CGFloat' to
> expected argument type 'Double'
>
> let v = (self * p).rounded(.towardZero) / p
> ^
> Double( )
Where a value is -0.16666666666674246, -0.0 is the rounded value to be
expected. However the current implementation returned -0.3333333333333.
This is because the floating point error. So this patch truncates it.
This problem arose after 6611ec8 commit. The root cause is linked to
this condition: `0 == layoutAdapter.offsetFromMostExpandedAnchor` at
line 589 in the method `Core.shouldScrollViewHandleTouch(_:point:velocity:)`.
If the value of `layoutAdapter.offsetFromMostExpandedAnchor` has a
floating point error, the condition evaluates to false. As a result,
the panel moves even when the tracking scroll view is intended to
scroll.
This problem may not occur if there is no floating point error.
This is the revised version of commit 448fc5c.
Commit 448fc5c has a critical regression in scroll tracking that can cause the
scroll content to bounce after moving a panel, for example, pulling down it from
full to half state.
By re-investigating #524, I found that this problem only occurred with the
`fitToBounds` content mode and a small scroll view content.
Therefore I fixed it in the more specific way.
UIScrollView would unexpectedly change its scroll offset after updating the bounces property when dealing with small scrollable content. This fixes issue #524, "Scrolling jumps when tableView content is small".
Ortwin Gentz, FutureTap
Shin Yamamoto