#22268 cmux NEXT: mouse buttons 4 and 5 go back and forward

Mouse buttons 4 and 5 now go back and forward in cmux NEXT, as they do in Chrome, Safari, Claude and Codex. Before this, nothing listened for them.

• Over a browser tab (its page or its toolbar), they walk that tab's history. They use the same BrowserChromeView command as browserBack / browserForward.
• Anywhere else, they run Go Back / Go Forward (focusHistoryBack / focusHistoryForward). That's the shown page's own history first (App Store, New Tab to chat), then the location trail, as plans/cmux-next/history.md 4.2b describes.
• A remote desktop in control mode keeps the buttons and sends them to its host, like every other button.
• A Chromium page draws in its own child window, so a click there arrives in that window. The tab is found in the cmux window that holds it.
• The press is consumed and the release navigates, as in Chrome.
• Debug Settings > Back and Forward > navigation.mouseSideButtons is on by default and turns this off.

The trackpad swipe (in progress, separate PR) uses the same browser path.

Changelog

Added: mouse buttons 4 and 5 go back and forward in browser tabs and through Go Back / Go Forward elsewhere.

Proof

Not UI: an input route with no new view. MouseSideButtonsTests cover:

• the button mapping
• the routing split
• the remote-desktop hit test

Observed / Not run

• Observed: the PR's CI compile and tests (see checks).
• Not run: a click with a physical mouse on a running build. No GUI runner has a mouse with side buttons, and debug.mouse can't synthesize buttons 4 and 5 yet.

🤖 Generated with Claude Code

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Summary by cubic

Mouse buttons 4 and 5 now go back and forward in cmux NEXT, matching Chrome, Safari, Claude, and Codex. Before this, nothing listened for them.

Details

• Over a browser tab they walk that tab's history via the same command as browserBack / browserForward; anywhere else they run Go Back / Go Forward.
• A remote desktop in control mode keeps the buttons and sends them to its host, like every other button.
• The release follows the route its press chose, so a press that went to a remote host never navigates after control mode changes or the pointer moves.
• The press is consumed and the release navigates, as in Chrome. A Chromium page draws in its own child window, so the tab is found in the cmux window that holds it.
• Debug Settings > Back and Forward > navigation.mouseSideButtons is on by default and turns this off.
• The regenerated defaults schema drops a stale key.agentPane.searchFiles entry and adds setting.browser.verificationAutofill.enabled.