[url_launcher][web] Link should work when triggered by keyboard

[mdebbar]

Background

You can think of the Link widget (on the web) as two components working together:

1. The <a> element created by the Link widget. This is essential to make all browser interactions feel natural (e.g. context menu, cmd+click, etc).
2. The children of Link widget. These are the widgets visible to the user (e.g. a button or a hyperlink text) and the user can interact with them the same way they would interact with any Flutter widgets (focus, pointer click, etc).

In order for the Link widget to navigate to a URI, the two components from above have to indicate their intent of navigation:

1. Some widget has to call followLink to indicate that the click successfully landed (i.e. hit tested) on it. E.g. if it's a button, then the onPressed callback should lead to a call to the Link's followLink.
2. The <a> element also has to receive an event to initiate the navigation.

The PR

We used to only handle click events on the <a> element, and no handling for keyboard events was present. So when a user tabs their way to the Link, then hits "Enter", the following happens:

1. The focused widget (e.g. button) that received the "Enter" will correctly indicate its intent to navigate by calling followLink.
2. The intent from the <a> element is lost because we were only handling clicks and not keyboard events.

This PR adds handling of keyboard events so that it works similar to clicks.

Fixes flutter/flutter#97863

[ditman]

LGTM! Thanks for adding this! Small question about attaching the keydown listener to the window, instead of the link element (or the view of the Widget)

[ditman]

Does this method need to check if the keydown is "enter", "space" or something like that? This'd trigger the link pressing any key, right?

[mdebbar]

I don't want to assume what keys the framework and/or app are using as a trigger for the link.

As I explained in the PR description, there are 2 conditions that have to be met in order for the link to navigate. The code in _onDomKeydown is only one part. The other part is the app/framework calling followLink when they decide that the key should trigger the link. Here's roughly the sequence of events:

1. User tabs through the app until the focus reaches the button wrapped by the Link widget.
   • (Keep in mind that this tabbing is happening purely in the framework; the browser has no idea what's focused)
2. User hits "Enter".
3. The framework received the keydown event.
   1. It decides to trigger the onPressed on the button (because the button has focus).
   2. The button's onPressed callback leads to a call to the Link's followLink.
   3. followLink calls LinkViewController.registerHitTest which sets _hitTestedViewId to the Link's platform view ID.
4. The LinkViewController also receives keydown event.
   1. It checks if _hitTestedViewId is already set (indicating that the framework already decided to trigger the link).
   2. If _hitTestedViewId is set, it navigates to that Link's URI.

[ditman]

(This should be part of the documentation of the widget :P)

[mdebbar]

Added as comments to the _onGlobalKeydown method since that's where this information is relevant.