The shared assert_layout_sane degenerate check flagged an interactive element
as 'off-viewport' when it sat below the fold at rest but was reachable by
scrolling an overflow-y:auto ancestor (tall dialog on a short viewport). This
produced a CI-environment-sensitive false positive on the Kanban board settings
modal at 480x320 German (the field is inside the .kanban-modal-overlay which is
overflow-y:auto), passing locally but failing on the CI runner's font render.
Add reachableByScrollableAncestor(): an off-viewport edge absorbed by a
scrollable (overflow auto/scroll, content-overflowing) ancestor is reachable and
not a defect. Genuinely-unreachable elements (no scroll escape, e.g. inside
overflow:hidden) are still flagged. +2 unit tests covering both directions.
nesquena-hermes