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.
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