And remove the now-redundant members m_scrubbing, m_scrubber_in_use,
and m_automatic_scrolling_kind.
This also made it clear that we weren't canceling the autoscroll
timer if the scrollbar got disabled while it was scrolling, so
this fixes that too.
Note that m_hovered_component is only updated on mouse move, not while
just keeping left down. It's arguably wrong to update it on mouse move
while the mouse is down, I'll probably change things so that it doesn't
update there either.
The behavior on click-in-gutter-keep-left-down-then-move-mouse varies
a surprising amount between platforms. This implements the macOS
behavior where the scrubber follows the mouse direction while scrolling
by pages. (To be precise, it's the macOS behavior of Finder and Preview,
Safari has Windows's scrollbar behavior).
On Windows, the first click locks in the scroll direction and then
dragging the mouse off the scrubber in that direction makes the
scroll continue, but dragging it off the other direction has no effect.
I see no reason for that behavior.
Rather than disable and re-enable the timer, always keep it active
and make it do collision checks to decide if it should have an effect.
This is because set_automatic_scrolling_active(true) calls the
timeout callback immediately before starting the timer, and
when clicking the gutter this callback could disable the timer
again (if the first page scroll put the scrubber under the cursor).
Intead of making set_automatic_scrolling_active() work when it's
called reentrantly (which is easy: just swap the order of
on_automatic_scrolling_timer_fired() and timer->start() so that
on_automatic_scrolling_timer_fired() can immediately stop the
timer again, but it's confusing), make the timer check if it
should do anything.
This is keyed off m_last_mouse_position instead of
m_hovered_component because m_hovered_component is a visual state
and we arguably shouldn't modify it while the left mouse button
is down (as it indicated what part is activated on click).
Most callers of set_automatic_scrolling_active() also change
m_automatic_scrolling_kind, and it makes it possible to make timer
behavior dependent on the autoscroll kind later.
In order to calculate a thumb size that is a representation
of the visible portion (page) of the content, that information
needs to be taken into account.
