A too-small chevron on a split button misses into the menu

What it is

The chevron of a split button is often drawn as a sliver beside the body, 8–12 pixels of visual. If the hit region follows the drawing, contacts aimed at Save regularly land on the chevron and the overflow opens instead. A too-small chevron does not mainly fail to open the menu. It fails to fire the default—the frequent action is forced through an extra menu.

Why it happens

In Fitts’s law, a skinny second target glued to a large first target eats the first target’s endpoint scatter. The wider the body and the thinner the chevron, the more scatter falls on the chevron—the opposite of the intuition that a wide body emphasizes default. Unless the chevron’s hit is deliberately widened and seamed off the body, emphasizing default raises glances off into overflow. A touch contact circle makes it worse. Once the menu is open by mistake, people dismiss and retry the body, slower than a separate More, and may hit a destructive item in the panic. A skinny chevron also fails the other way: people who mean to open the menu miss, and conclude there is no overflow. One thin sliver manufactures both false opens and failed opens.

Studying it

Hold body width fixed and vary the chevron’s visual width and hit width (they may differ). The task is repeated default clicks with occasional overflow clicks. Log which region the landing hits.

Independent variables: chevron visual width, chevron hit width, input channel, body width. Dependent variables: default intent that opened the menu, overflow intent that hit the body, time across two attempts.

Measuring only “can the menu be opened” treats enlarging the chevron as the sole goal and ignores false default opens. Count both errors.

Where it stops holding

A precise desktop pointer can live with a visually narrower chevron, but hit should still be wider than look. Keyboard can Tab body and chevron as two stops, dodging hit geometry, with two names (Save / more save options). A menu button (the whole control opens a menu, no default click) has none of this miss; if touch cannot split well, fall back to a menu button or two independent keys.

Applying it

• Give the chevron a hit region at least as large as the minimum reach. The drawing may be a bit narrower; the hit must not be a slit.
• Leave a non-overlapping seam between body and chevron hits so an edge tap is not arbitrated.
• A false menu open should dismiss on Escape or outside click without running a menu item.
• Verify with fifty aimed Saves on touch. Menu opens should be near zero; aimed chevron taps should open stably. One side high and the other low means chevron geometry has failed.

Related

• Within the group: E1.14.1 A button group presents related actions that are exclusive or parallel · E1.14.2 A split button separates default and overflow onto body and chevron · E1.14.4 Too many items in a group should become another control, not a longer row
• Adjacent: E1.07 Button hit targets · B1.01 Fitts’s law · C2.03 Minimum spacing of touch targets
• Search terms: split button · chevron hit target · Fitts's law