diff --git a/docs/buzz-help.md b/docs/buzz-help.md
index e29914ce89..429d22ea0c 100644
--- a/docs/buzz-help.md
+++ b/docs/buzz-help.md
@@ -365,7 +365,8 @@ revokes all of them, and a pairing unused for 30 days expires.
   you change most of them and also shows the fixed message-formatting keys. In
   an empty composer, Up selects a message unless **Edit last message with Up Arrow**
   is on. With a message selected, E edits it, R reacts, T or → replies in thread,
-  U marks it unread, and Delete deletes your own message.
+  U marks it unread, and Delete deletes your own message. Option-click a message
+  (Alt-click on Windows and Linux) to mark it unread, selected or not.
   ⌘1–9 switch communities, and ⇧Esc marks everything read. If a plugin uses
   the same key as another shortcut, Settings → Shortcuts marks it "Also used
   by"; change one of them to choose which fires. In the Mac app, **Buzz →
diff --git a/src/app/message-shortcuts.test.ts b/src/app/message-shortcuts.test.ts
index f3ee5629e6..302298c0ba 100644
--- a/src/app/message-shortcuts.test.ts
+++ b/src/app/message-shortcuts.test.ts
@@ -78,6 +78,61 @@ it("deletes, marks unread and replies to the selected message with Slack's keys"
   expect(reply).toHaveBeenCalledTimes(2);
 });
 
+it("marks a message unread with Option-click, but not from its controls, media, code, day divider or a text selection", () => {
+  const { controller, reply } = setup(true);
+  (document.activeElement as HTMLElement | null)?.blur();
+  const row = document.querySelector<HTMLElement>("[data-message-id]");
+  row?.insertAdjacentHTML(
+    "afterbegin",
+    "<div><span data-day>Today</span></div>",
+  );
+  row
+    ?.querySelector("[data-layout]")
+    ?.insertAdjacentHTML(
+      "beforeend",
+      '<p>Text <a href="#one">link</a> <img data-copy-emoji=":wave:" alt=":wave:"></p>' +
+        '<img alt=""><pre>code</pre>' +
+        '<div role="group" aria-label="Reactions"><button>👍</button></div>',
+    );
+  const text = row?.querySelector("p");
+  const click = (selector: string, init: MouseEventInit = { altKey: true }) =>
+    row
+      ?.querySelector(selector)
+      ?.dispatchEvent(
+        new MouseEvent("click", { bubbles: true, cancelable: true, ...init }),
+      );
+  click("p", {});
+  for (const modifier of ["shiftKey", "metaKey", "ctrlKey"])
+    click("p", { altKey: true, [modifier]: true });
+  for (const selector of [
+    "a",
+    "img:not([data-copy-emoji])",
+    "pre",
+    "[aria-label='Reactions']",
+    "[aria-label='Reply']",
+    "[data-day]",
+  ])
+    click(selector);
+  expect(reply).toHaveBeenCalledTimes(1);
+  // The end of a drag, including WebKit's Option-drag, leaves text selected.
+  const range = document.createRange();
+  if (text) range.selectNodeContents(text);
+  document.getSelection()?.addRange(range);
+  click("p");
+  document.getSelection()?.removeAllRanges();
+  text?.addEventListener("click", (event) => event.preventDefault(), {
+    once: true,
+  });
+  click("p");
+  vi.spyOn(controller, "canMarkUnread").mockReturnValueOnce(false);
+  click("p");
+  expect(controller.markUnread).not.toHaveBeenCalled();
+  // A custom emoji is part of the text, not media.
+  click("[data-copy-emoji]");
+  expect(controller.markUnread).toHaveBeenCalledExactlyOnceWith({ id: "one" });
+  expect(document.activeElement).toBe(document.body);
+});
+
 it("leaves Delete alone on someone else's message and without a selection", () => {
   const { controller } = setup(false);
   expect(press("Backspace")).toBe(false);
diff --git a/src/app/shortcuts.ts b/src/app/shortcuts.ts
index 7ef145dd0c..7d571ab7fe 100644
--- a/src/app/shortcuts.ts
+++ b/src/app/shortcuts.ts
@@ -5,6 +5,7 @@ import type { Appearance } from "../shared/theme/service";
 import { statusShortcut } from "../features/user-status/status-shortcut";
 import {
   activeMessageList,
+  markUnreadOnAltClick,
   reactionTrigger,
   replyTrigger,
   selectedMessage,
@@ -203,6 +204,9 @@ export function registerAppShortcuts(
   const settings = () => shortcuts.runHost("settings");
   window.addEventListener("buzz:open-settings", settings);
   remove.push(() => window.removeEventListener("buzz:open-settings", settings));
+  // After React's handlers, so clicks they stop or prevent are left alone.
+  window.addEventListener("click", markUnreadOnAltClick);
+  remove.push(() => window.removeEventListener("click", markUnreadOnAltClick));
   return () => {
     for (const dispose of remove) dispose();
   };
diff --git a/src/features/messages/MessageRow.test.tsx b/src/features/messages/MessageRow.test.tsx
index 8075fadb24..912656d3b8 100644
--- a/src/features/messages/MessageRow.test.tsx
+++ b/src/features/messages/MessageRow.test.tsx
@@ -85,6 +85,9 @@ it("discloses an automatic failure reply while keeping its original text availab
     expect(details).toHaveAccessibleName(
       "Spongebob: Couldn’t finish · Hide details",
     );
+    // Option-click marks the message unread, so it leaves the details open.
+    fireEvent.click(screen.getByText(content), { altKey: true });
+    expect(details).toHaveAttribute("aria-expanded", "true");
     fireEvent.click(details);
     expect(details).toHaveAttribute("aria-expanded", "false");
     view.rerender(
diff --git a/src/features/messages/MessageRow.tsx b/src/features/messages/MessageRow.tsx
index 9bd3dcf6dc..73add6cf14 100644
--- a/src/features/messages/MessageRow.tsx
+++ b/src/features/messages/MessageRow.tsx
@@ -1698,11 +1698,13 @@ function AgentsMessageRow(props: MessageRowProps & { failure?: boolean }) {
             // button: revealing an anchor takes it out of the Tab order.
             data-message-id={row.id}
             // Open, a click anywhere in the box folds it, except on its own
-            // controls and links or at the end of a text selection.
+            // controls and links, at the end of a text selection, or with
+            // Option, which is Mark unread's gesture (message-selection.ts).
             onClick={(event) => {
               const target = event.target as Element;
               if (
                 !open ||
+                event.altKey ||
                 !event.currentTarget.contains(target) ||
                 target.closest(
                   "a, button, input, textarea, select, summary, [role='button'], [role='menuitem'], [contenteditable='true']",
diff --git a/src/features/messages/message-selection.ts b/src/features/messages/message-selection.ts
index 9af3faff06..5720a446d7 100644
--- a/src/features/messages/message-selection.ts
+++ b/src/features/messages/message-selection.ts
@@ -193,3 +193,29 @@ export function moveMessageSelection(event: KeyboardEvent<HTMLElement>) {
     focusWithKeyboard(composer);
   }
 }
+
+const OWN_CLICKS =
+  "a, button, input, textarea, select, summary, label, [role='button'], [role='menuitem'], [role='group'], [contenteditable='true'], img:not([data-copy-emoji]), video, audio, pre, [data-copy-code]";
+
+/** Slack's Option-click marks a message unread, as U does, from its box only:
+ * not its day divider, controls, media, code or reactions, nor the end of a
+ * text selection, which WebKit's Option-drag also makes. */
+export function markUnreadOnAltClick(event: MouseEvent) {
+  const { target } = event;
+  if (!event.altKey || event.ctrlKey || event.metaKey || event.shiftKey) return;
+  if (event.defaultPrevented || !(target instanceof Element)) return;
+  const element = target.closest<HTMLElement>(ROW);
+  if (!element) return;
+  const box = focusTarget(element);
+  if (
+    !box.contains(target) ||
+    box.contains(target.closest(OWN_CLICKS)) ||
+    !target.ownerDocument.getSelection()?.isCollapsed
+  )
+    return;
+  const owner = listFor(element);
+  const list = owner && lists.get(owner);
+  const id = element.dataset.messageId;
+  const row = id ? list?.row(id) : undefined;
+  if (row && list?.canMarkUnread(row)) list.markUnread(row);
+}