A menu taller than the room under the menu bar scrolls inside an egui
ScrollArea whose floating scrollbar is drawn over the rows' right edge,
where the shortcut text ends. At 1.0x the 2-point handle grazes the last
glyph; at 1.75x the bar is in its wide hovered state with a semi-opaque
background and hides the last character of every shortcut, which reads as
clipping. The scroll content, its clip and the rows are the level's width
in both modes (instrumented), so nothing else needs to move: the scroll
area hides its bar. The up/down strips, the wheel and dragging were already
the designed ways to scroll a menu, like native menus; the native macOS
menu bar does not go through this code.
Test: tests_menubar::a_scrolled_menu_draws_no_scrollbar_over_its_shortcuts
(a 700 pt and a 1400 pt window: no tall narrow rectangle along the level's
right edge, equal widths), via a new HeadlessView::painted_shapes; fails on
main with the bar's track [512, 59]-[514, 660] and handle.
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