This is on linux with:

• Thunderbird daily 112.0a1 (2023-02-27) (64-bit)
• at-spi2-core 2.46.0
• python-atspi 2.46.0

However I expect this to be reproduceable on all the platforms

The problem

It's possible to use up and down arrow keys to navigate in the new message list introduced in Thunderbird 111.
It's also possible to select more than a single message in this list either by holding shift key and navigating in any direction or holding ctrl key and selecting individual messages by pressing the space key.
When extending or shrinking the selection this is not communicated to assistive tools. When using this new message list with screen readers, I dont know which messages are selected when holding down ctrl or shift key.

Steps to reproduce

• Launch thunderbird, use tab or shift+tab or F6 and shift+F6 to focus the message list.
• Press page up or page down key so you start somewhere in the middle of the list not at the top or at the bottom.
• Press and hold down shift key and start slowly moving with the down arrow key.

Actual results

Multiple message selection is expanding however the selection state of the messages is not communicated to assistive tools when selection state for individual messages is changing. The focus is moving though.

Expected

The fact message is selected or not selected should be communicated to assistive tools.

Somewhat related to bug 1813492 since we should allow properly reading the message.
Once we fix that, we can improve this.

What would be the most natural and helpful string to add?
Something like "3 messages selected. message 1: subject, message 2: subject, message 3: subject"?
Something shorter? Something more verbose?

I would say no more localized strings should be invented while fixing this one. This should be wired into appropriate platform APIs to signal a selected state where available. The control should also fire platform specific selection changed events as it happens.
See the [https://www.w3.org/WAI/ARIA/apg/practices/keyboard-interface/#kbd_focus_vs_selection](Focus VS Selection and the Perception of Dual Focus) and [https://www.w3.org/WAI/ARIA/apg/practices/keyboard-interface/#kbd_selection_follows_focus](Deciding When to Make Selection Automatically Follow Focus) to better understand what I am looking for.
Also please look at [https://www.w3.org/WAI/ARIA/apg/patterns/listbox/](listbox pattern) and pay attention to the recommended states. I think aria-selected and aria-multiselectable is what we might need.

This was fixed by bug 1820604