Scrolling

Scrollers, scrollbars, and the ways a scroll can be driven.

overflow: auto | scroll makes a box scroll on whichever axis its content overflows: vertical, horizontal, or both. It scrolls by:

• wheel (Shift+wheel, or a horizontal wheel, scrolls sideways),
• dragging a scrollbar thumb,
• touch: a finger drags the content itself,
• keyboard: arrows, PageUp/PageDown, Home/End scroll the box under the pointer, when no input has focus.

Scrollbars are an overlay on the box's trailing edge: they appear only on an axis that actually has travel, the thumb is the box's fraction of the content (to a grabbable floor), and when both axes scroll the tracks stop short of the corner. They are drawn over the content, because a scroller clips its children so they can't be part of the subtree, and drawn from the same geometry the drag hit-tests, so they can't disagree.

Scroll-into-view runs on Tab: focusing something below the fold scrolls its box far enough to show it (typing in a textarea does the same for the caret).

Offsets live in the shell keyed by the scroller's index in tree order, so they survive the whole-tree rebuild, so tapping a row doesn't scroll the list to the top. A press on a thumb never becomes a tap on the content beneath it.

Not done: no click-on-track paging, no kinetic/inertial touch fling, no scrollbar hover/fade states, no scrollbar-width/scrollbar-color, no overscroll-behavior, and overflow-x/overflow-y can't yet differ (one overflow governs both axes).