Compose's focus system handles keyboard, D-pad and accessibility navigation. The APIs are small, but modifier order matters, and a form with no focus handling is unusable on any device without a touchscreen.
NestedScrollConnection gives parents four chances to consume scroll deltas around a child. Choosing the wrong one is why collapsing headers collapse late, and why same-axis nesting misbehaves.
Multi-touch gestures produce pan, zoom and rotation simultaneously. The centroid — the point between the fingers — is what makes the transform feel anchored, and ignoring it is the most common photo-viewer bug.
Hand-rolled swipe gestures compare a distance threshold and ignore velocity, which is why a fast flick feels wrong. AnchoredDraggableState models discrete positions and settles between them using both distance and speed.
Modifier.clickable adds accessibility semantics, focus, keyboard activation, ripple and a minimum touch target on top of tap detection. Reaching for detectTapGestures instead silently drops all of it.
pointerInput gives you a coroutine scope over raw pointer events. Understanding that gestures are suspending loops rather than callbacks explains the whole API, including why the key parameter matters more than it looks.
Compose's accessibility checks catch touch targets, contrast and missing labels automatically. The failures they cannot detect — wrong labels, bad order, gesture-only actions — need a person, and knowing the split makes both cheaper.
Screen readers traverse in layout order, which breaks when a floating action button, a sticky header or a two-column layout puts visual priority somewhere else. Traversal groups and indices reorder without moving anything.
A composite component produces one semantics node per child by default, so a card becomes four swipes. mergeDescendants, clearAndSetSemantics and invisibleToUser control the shape of the tree.
Every Compose UI produces a semantics tree alongside the layout tree. It's what screen readers read, what tests query, and what autofill inspects — so getting it right serves three audiences at once.
Compose tests idle-wait by default, which makes animated UI either flaky or untested. Disabling autoAdvance gives you frame-level control, turning 'wait and hope' into deterministic assertions at chosen moments.
Shared element transitions match two composables by key and animate between their bounds. Understanding that neither element travels — a third overlay draws the motion — explains the scope requirements and the common failures.
