# Common pitfalls

## Competing geometry owners

A layout, anchors, and explicit size bindings can overwrite one another.
Name the owner of each axis and remove the competitors.

`childrenRect` is not inherently a binding loop.
It loops only when the children's actual geometry depends on the parent size being derived from that same `childrenRect`.

## Accidental binding replacement

An imperative assignment to a bound property removes its binding.
Update the binding's source property instead, or intentionally install a new binding with `Qt.binding(...)`.

For a Slider bound to `model.volume`, `onMoved: model.volume = value` updates the dependency and preserves the Slider's `value` binding.
Avoid `onValueChanged` for user intent because it also fires for programmatic changes.

## Hidden structural coupling

`parent.parent` reaches through implementation details.
Use an id within one component and required properties or signals across component boundaries.

## Plain JavaScript mutation

Bindings do not observe deep changes such as `config.theme = "light"` when `config` is a plain JavaScript object.
Replace `config`, use typed QML properties, or use an observable model.

## Input without semantics

A `TapHandler` or `MouseArea` does not make a custom item a complete control.
Prefer Qt Quick Controls.
If a custom interactive item is necessary, provide keyboard activation, active focus, visible focus, an accessible role, and an accessible name.

## Zero-size visual roots

`Item` has no useful implicit size by default.
A reusable visual component must expose a meaningful implicit size or receive actual geometry from its owner.

## Wrong validation claim

A successful `qmllint` run proves static checks only.
An offscreen startup proves loading and instantiation only.
Neither proves pointer, keyboard, focus, accessibility, animation, or visual behavior.