Accessibility
role="region" with tabindex="0" named by scrollRegionLabel. Development builds warn once if the label is missing in that case. Keyboard scrolling (arrow keys, Page Up and Down, Home and End) is the browser's own, on the native scroll root.
Pass focusable to make the region a tab stop whenever it overflows, even when it holds focusable descendants, so a reader can scroll it before tabbing through the controls in it. The type then requires scrollRegionLabel or aria-labelledby, so a focusable region always has an accessible name. A region that does not overflow is never a tab stop, so it adds no empty stop to the tab order. tabIndex is not a prop.
The overlay thumb and its track are aria-hidden, take no focus, and expose no role. Pointer, wheel, touch and keyboard scrolling all go through the native scroller. Under forced colors the system scrollbar stays and no overlay thumb is drawn. The edge fades are a mask on the scroller, so they change no text, role or reading order.
While a block fade is on, the scroller reserves the fade's 28px as scroll padding, so a control that focus scrolls into view lands clear of the fade (WCAG 2.4.11). A sticky part inside the scroller registers with it, and the scroller reserves the larger of the part's reach and the fade at that edge. The focus ring of a focusable region is drawn by the frame, so the mask never clips it.
scrollbarGutter changes layout only. A stable gutter keeps the content width the same whether or not the region scrolls, so content does not shift sideways when it starts to overflow, and text at the inline end is never under the scrollbar.