fix: announce sending status to screen readers via live region

[isherstneva]

When a user submits a message, the activity enters the "sending" state while awaiting server acknowledgement. The visual "Sending" indicator was rendered next to the activity but not inside an ARIA live region, so screen readers never announced it.

Adds LiveRegionSendSending component (mirroring LiveRegionSendFailed) that watches for activities newly entering the "sending" state and queues the localized "Sending message." string into the polite live region. Also adds TRANSCRIPT_LIVE_REGION_SEND_SENDING_ALT localization key and a corresponding integration test.

Bug ticket: https://msazure.visualstudio.com/CCI/_workitems/edit/36003607/

Fixes #

Changelog Entry

Description

Design

Specific Changes

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

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[Copilot]

Pull request overview

Adds ARIA live-region announcements for the “sending” state so screen readers narrate “Sending message.” when an outgoing activity is awaiting server acknowledgement.

Changes:

• Added LiveRegionSendSending component and mounted it in LiveRegionTranscript.
• Introduced new localization key TRANSCRIPT_LIVE_REGION_SEND_SENDING_ALT.
• Added an accessibility integration test covering the new live-region narration.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/component/src/Transcript/LiveRegionTranscript.tsx	Mounts the new sending live-region component alongside the existing send-failed narration.
packages/component/src/Transcript/LiveRegion/SendSending.tsx	Implements detection of activities newly entering the SENDING state and queues localized narration.
packages/api/src/localization/en-US.json	Adds the localized “Sending message.” string and comment for translators.
tests/html2/accessibility/liveRegion/activityStatus.sendSending.html	Adds an integration test asserting the live region narrates “Sending message.” for unacknowledged outgoing activities.
CHANGELOG.md	Documents the accessibility fix in the release notes.

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[Copilot]

prevActivityKeysOfSending.has(key) will throw if usePrevious returns undefined on the initial render (common for “previous value” hooks). Consider defaulting prevActivityKeysOfSending to an empty Set (or handling the undefined case inside the memo) so the first render cannot crash. Also, useLiveRegion(() => hasNewSending && liveRegionSendSendingAlt, ...) returns false when not sending; if useLiveRegion expects a string/undefined, prefer returning undefined/null rather than a boolean to avoid accidental narration/queuing of a non-string value.

[Copilot]

Array.from(sendStatusByActivityKey) allocates an intermediate array on every recomputation. Since sendStatusByActivityKey is already iterable, iterating it directly (e.g., a for...of loop) avoids the extra allocation and can reduce overhead for large transcripts.

Suggested change

	() =>
	Array.from(sendStatusByActivityKey).reduce(
	(activityKeysOfSending, [key, sendStatus]) =>
	sendStatus === SENDING && !isPresentational(getActivityByKey(key))
	? activityKeysOfSending.add(key)
	: activityKeysOfSending,
	new Set<string>()
	),
	() => {
	const activityKeysOfSending = new Set<string>();
	for (const [key, sendStatus] of sendStatusByActivityKey) {
	if (sendStatus === SENDING && !isPresentational(getActivityByKey(key))) {
	activityKeysOfSending.add(key);
	}
	}
	return activityKeysOfSending;
	},

[Copilot]

Correct the wording “screen reader” → “screen readers” for grammatical correctness in the translator comment.

Suggested change

	"_TRANSCRIPT_LIVE_REGION_SEND_SENDING_ALT.comment": "This is for screen reader. When the user sends a message, the live region will announce this string to indicate the message is being sent.",
	"_TRANSCRIPT_LIVE_REGION_SEND_SENDING_ALT.comment": "This is for screen readers. When the user sends a message, the live region will announce this string to indicate the message is being sent.",