When the target of a mousewheel event is removed, it breaks velocity
scrolling.
Previously, we were preserving the entire screen range when scrolling
with the mouse wheel, which caused a lot of DOM nodes to accumulate. Now
we only preserve the individual line and line number associated with the
target of the mousewheel event, moving them just off screen below all
the on-screen lines and line numbers. This keeps the number of DOM nodes
limited while retaining velocity effects.
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