Function shift_wheel_remap 

pub fn shift_wheel_remap(
    event: &WidgetEvent,
    ctx: &EventContext<'_>,
) -> Option<WidgetEvent>

Rewrite a Shift+wheel notch into a horizontal one, or decline.

A vertical-only wheel held with Shift scrolls a horizontally-scrollable surface sideways — the convention TabBar established in this crate and every desktop toolkit shares. The transform is a delta rewrite, which handle_scroll_event cannot express because it reads the delta off the event; a surface that wants it builds the rewritten event here and hands that to the shared handler from its ScrollableBehavior::before arm, so the arithmetic is still written once.

Declines — returning None, meaning “no remap, treat this event as it came” — for anything but a wheel-family [WidgetEvent::Scroll] held with Shift whose horizontal component is zero. Two of those clauses carry the rule rather than the example:

• A delta with a real horizontal component is a trackpad’s own two-axis stream, and rewriting it would throw the axis the user actually moved.
• A [ScrollSource::TouchPan] is never remapped. A finger has no Shift key, so the modifier could only arrive from a keyboard held during a pan, and turning that pan sideways is not what the hand asked for.