The mixed appearance must separate from checked
Aliases: checkbox dash · mixed versus ticked · indeterminate glyph
What it is
Mixed must come apart both from an empty box and from a tick. The usual means is a dash inside the box, or a fill smaller than full, not a paler tick or a half-opacity fill. When distinct from checked fails, partial selection is scanned as “all selected,” and people stop inspecting children under a folded parent. If empty and mixed differ by a few pixels of grey, mixed is scanned as “nothing selected.”
Separation rides on shape, not hue. In colour-blind views and greyscale screenshots the shape remains; a pale blue tick does not.
Why it happens
A tick is a long-trained “true” sign. Any stroke inside the box that is close to a tick is classified as true. A faint tick, a tick missing a stroke, or a translucent tick still lives in the tick family — only harder to see. A dash, a square, a fill with the diagonal removed, sit outside that family and can occupy the third bin.
Scanning a folded tree is a three-way classification, not a reading of each box’s legend: empty / mixed / full. Pairwise distance among the three must beat noise. Full already owns a high-contrast interior mark; mixed cannot borrow that mark and “weaken” it — weakening only drops detectability, it does not add a category.
Where it stops holding
A white dash on dark fill and a dark dash on light fill do not share one opacity. High-contrast modes that thicken every interior mark into similar strokes can collide dash and tick again; that mode needs a still-separable geometry (a shorter dash plus a hollow box). After print loses colour, shape must still split. An animation from mixed to fully ticked that passes through a frame that looks like a faint tick will be misread as already-all; skip the tick family in the in-between. At tiny icon sizes (deep indent in a tree) a dash may blob into a dot; enlarge the box rather than retreat to a pale tick.
Applying it
- Draw mixed as a dash or an interior that is clearly not a tick; ban pale ticks, translucent ticks, and ticks with a missing stroke.
- Scan a tree that contains empty, mixed, and full in greyscale, simulated colour-blindness, and high contrast; the three must still split.
- Do not let colour be the only difference; fill may help, but shape has to stand first.
- How to check: at real tree-node size, give someone two seconds to mark the mixed ones. Any marked as full or as empty failed to separate.