RFC created: 2020/01/15
Last updated: 2020/01/15
This RFC has been merged
# Focus Behavior
When discussing focus behavior, we need to consider the three ways you can focus on elements: click focus, programmatic focus, and sequential focus. In general, anything that is click focusable or sequentially focusable is programmatically focusable, and focus behavior for clicks and sequential navigation can depend on the user agent and system settings. For example, Safari will not apply focus on buttons when clicked, but it will apply focus on buttons when the focus method is invoked .
As such, the set of click focusable and sequentially focusable elements is not something that we can know a priori. What we do know when observing our supported user agent matrix is that, the set of potentially focusable elements is the same. This proposal makes the assumption that it is acceptable for the framework to introduce shadow semantics via various focus management polyfills--even if their behavior contradicts user settings--as long as the access to all potentially focusable elements is preserved.
Generally speaking, it is outside the scope of LWC to normalize different behaviors across its supported user agent matrix. However, in the case of focus delegation, the framework does need to make informed decisions to normalize focus behaviors in order to avoid a broken user experience. This is especially true in the case of sequential focus navigation.
The primary goal of this proposal will be to minimize differences between its synthetic Shadow DOM and native Shadow DOM implementations to facilitate the transition of components to native Shadow DOM.
# Detailed design
For simplification, unless otherwise stated, it can be assumed that all components are delegating focus.
For simplification, subsequent usage of the term "focusable" will refer to
whether or not an element is programmatically focusable (i.e., whether or not
an element can receive focus when its focus method is invoked or it has the
focusable = programmatically-focusable = click-focusable ∪ sequentially-focusable
# Focusable elements
The set of sequentially focusable elements is currently defined by this monster selector. This has been sufficient for our needs so far but should be reviewed and improved . We should go through the exercise of checking it against the results of focusable elements listed on this test page.
The set of click focusable and programmatic focusable elements is a superset of the set of sequentially focusable elements, and should include all elements with tabindex values of -1.
# Programmatic focus
In the case of programmatic focus on a custom element, if the custom element is a shadow-including ancestor of the currently focused element, then we don't do anything and the focused element should remain focused. Otherwise, the specification states that the first programmatically focusable element should receive focus. Since there is no DOM API that identifies such an element for us, we can instead:
- Invoke the focus method on the first element in our set of focusable elements, where the set of focusable elements is the union of the set of sequentially focusable elements and the set of elements with a negative tabindex content attribute value.
- Verify that the element received focus via
- If the element did not recieve focus then keep invoking the focus method on subsequent elements until we verify that focus was transferred.
The verification step is useful as a workaround for implementation bugs around
elements that should be focusable but aren't (e.g., in Safari,
area with no
href is click focusable and sequentially focusable but not programmatically
focusable) and is worth adding since it's simple and cheap.
Note that if a custom element without tabindex has no focusable elements in its shadow, then invoking the focus method should result in a NOOP.
# Click focus
In the case of click focusability, we can pretty much let the browser do its thing. The only case we need to handle is the one where the user directly clicks on the host element itself. When this happens, the specification says that the first click focusable element should receive focus. Since there is no DOM API that tells us what is click focusable, the next best thing would be to fall back to invoking the focus method on the host element.
# Sequential focus
In practice, sequential focus is the class of focusability for which behavior varies the most between user agents. For example, macOS users are able to customize the ability to skip certain focusable elements via a combination of system settings and browser settings.
In this proposal, we choose to ignore such settings in order to implement a sequential focus polyfill for LWC components which delegate focus. Non-Safari browsers on macOS also do this so there is precedence. Our existing implementation already does this (delegates focus RFC) but it is worth reviewing our coverage of focusable elements since we will be relying on that for all three classifications of focus.
The biggest drawback to this approach is the difference in the focus behavior when comparing synthetic and native Shadow DOM. For example, in LWC's synthetic shadow, a Safari user would have to tab through all focusable areas of a component regardless of whether they configured their system to skip certain elements. In native shadow, the same user with the same system configuration would be tabbing through a smaller set of focusable areas.
No alternatives have been considered. This proposal is simply an attempt to better align the framework with specifications.
# Adoption strategy
Existing LWC components that have already implemented a focus method can optionally remove their implementation in favor of the one provided by the framework. Keeping the existing implementation in place will not break existing functionality. Our recommendation that users implement their own focus method until the specs around this area became well-defined, has paid off!
# How we teach this
There is nothing unique to LWC that we need to teach. Developers can reference the WHATWG interaction specification to learn how focus works.
# Unresolved questions
- What should happen when a component that is delegating focus, has a tabindex value, and contains no focusable elements, is focused?
: Safari will actually remove focus from a focused button if you click it.
: The existing
[href] selector is too broad and will pick up non-focusable elements such as