Bounding-box simplification can make neighboring irregular targets overlap

What it is

Two visually staggered stars, two diagonal arrows, a row of bubble tails can have disjoint outlines whose AABBs still bite each other’s corners. A contact in the overlap is handed to one target by paint order, z-order, or traversal order, and the graphics do not tell the user which. That is overlap from simplification, not two deliberately expanded hot zones crowding each other.

Why it happens

Each irregular glyph’s AABB is pushed to extreme pixels on all four axes. If neighboring glyphs overlap in x or y projection, the rectangles intersect even when the true paths still have a gap. Toolkit hit-testing that meets overlap usually returns the topmost or first-hit node, so the overlap becomes a lottery the user cannot see. Trailing icons in list rows, badges on cards, and the needle tips of map pins are frequent sites: visual design left a gap along the outline, input wrapped bounds, and the gap vanished. After scroll and transform, overlap area also breathes with rotation, so the same pair conflicts on some frames and not others.

Studying it

Lay paired irregular icons out by visual gap (minimum outline-to-outline distance) and collect, under path hitting versus AABB hitting, which target owns taps that land in the “seam.” Independent variables are gap in pixels, rotation, z-order, and whether the pair lives in a scroller. Dependent measures are overlap area, mis-attribution rate, and how often people say “I meant the other one.” A control set of rectangular buttons at the same gaps should barely overlap, pinning the fault on simplification rather than spacing.

Where it stops holding

Deliberately enlarging hit regions to a minimum size until two rectangles meet is a different overlap, fixed by shrinking the expansion or adding space. Here the rectangles already intersect at visual size. Exact path hitting makes the claim false. An application that hits the parent and then clips polygons can untangle overlap itself; that is not default toolkit behavior. Two graphics that are meant to pass through each other in an animation overlap as content, not as a simplification bug.

Applying it

• Paint outline and AABB together in a debug layer. For pairs whose AABBs intersect while outlines do not, switch hitting to hull/path, or subtract the intersection from one hot zone.
• Do not let badges and trailing buttons respond through a parent’s large bounds; give them disjoint hot zones, and comment the z-order so a later paint-order change does not silently change hits.
• Accept with paired irregular icons: tap the seam between shapes and record which one fires; it must not drift with z-order.

Related

• Same group: C2.20.1 Hit regions need not match the visual shape of a control · C2.20.2 Irregular visuals are often hit-tested as bounding rectangles · C2.20.4 Hit shapes should follow where fingers actually land
• Nearby: C2.14 Separating touch targets from visual boundaries · C2.03 Minimum touch target spacing
• Search terms: overlapping hit targets · z-order · AABB intersection