[cheng-block]

Why

In Slack, Option-clicking a message marks it unread. In Buzz it did nothing, because nothing listened for a modified click on a message. You had to select it and press U, or use its menu.

What

Option-click (Alt-click on Windows and Linux) on a channel or thread message now marks it unread, like U.

How

• markUnreadOnAltClick in message-selection.ts finds the message the way U does and runs the same canMarkUnread and markUnread pair.
• It runs on the window after React's handlers. It skips controls, links, reactions, media other than custom emoji, code, the day divider, text selections, and prevented clicks.
• An open agent row folds on any click, so the fold now ignores Option-clicks.

Risk

Low. Only an Option-click with no other modifier on a message box does anything new.

• U is unchanged: it marks the message and its loaded replies, not everything after it as Slack does.
• Left out: Activity rows, media comments, and a Settings → Shortcuts row (it lists keys only).
• Any click still reselects a message selected earlier with the keyboard, as on main.
• feat(messages): quote selected message text into the composer #2392 adds a sentence after the U sentence in docs/buzz-help.md. The second to land needs a one-line merge.
• If Keep replies inline with immediate previews and preserved drafts #2610 (draft, hold-to-reply) lands, a slow Option-click could also pick a reply target unless the hold ignores modified presses.

Testing

• jsdom does not run WebKit's real Option-drag or mousedown focus. In-app step 4 covers the drag.

In-app check

Verified in the macOS app by Cheng. Alt-click on Windows and Linux was not tried; some Linux window managers take it.

Build: this PR at 814feaef8, merged locally with my other open PRs onto main at c037057a1. It ran as a debug app bundle (BUZZ_DEV_VIEWER= just desktop-bundle).

In a channel with read messages from other people:

1. Option-click a read message's text. Its … menu now offers Mark read.
2. Switch channels. The one you left stays bold until you open it.
3. Option-click a link, image, reaction, code block, author name, and Reply. Each acts as usual.
4. Option-drag across words. They stay selected, and the message stays read.
5. Plain, Shift-Option, and Command-Option clicks mark nothing.
6. Option-click a thread reply, and a custom emoji. Both now offer Mark read.

buzz-review-completed