ui: fix removed reasoning menu in single model mode on desktop

[benbenbenbenbenben]

Overview

This PR restores the missing reasoning menu in the web UI on large/desktop layouts when running in single model mode. The regression appears to have been introduced in #27744 and fixes #27981

Additional information

N/A

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: Qwen 3.8 27B was used to find the offending commit, I accepted a single line change suggestion and ran all git and gh operations manually.

[allozaur]

@benbenbenbenbenben please run npm run format in tools/ui and update the failing Storybook test

[ServeurpersoCom]

Thanks for the fix. Could you also test it in router mode? The models selector already mounts the reasoning submenu there, so this needs to not end up showing it twice before it can be merged. Also you need to launch "npm run format" before commit.

[Remowylliams]

Very looking forward to this PR making it through. Reasoning menu was super useful.

[benbenbenbenbenben]

Thanks for the fix. Could you also test it in router mode? The models selector already mounts the reasoning submenu there, so this needs to not end up showing it twice before it can be merged. Also you need to launch "npm run format" before commit.

I've tested it. In router mode on large screens the reasoning menu is in the correct location and I confirmed it doesn't duplicated when the screen is narrow.

Model selector on large screens:

Model selector on small screens:

Reasoning in "+" menu on small screens:

[DocTam]

Additional Confirmation & Regression Timeline

I have tested multiple builds and can provide a clear regression timeline for this issue:

• Build B10621 (Last known good): Reasoning menu works perfectly on Desktop layout.
• Build B10687 (First known bad): Reasoning menu disappears on Desktop layout.
• Build B10809: Same behavior, menu is missing.
• Build B10930 (Latest): Issue still persists.

Technical Details:

• When the browser window width is >= 768px (Desktop layout), the Reasoning menu is missing.
• When the browser window width is <= 767px (Mobile layout), the Reasoning menu reappears.
• This perfectly matches the standard CSS md breakpoint (Tailwind). The regression introduced in PR ui: Improve Chat Form Actions UI/UX (models selector, add panel) #27746 is specifically tied to the desktop viewport rendering logic.

Workaround:
Resizing the browser window to <= 767px to trigger the mobile layout still works perfectly, which confirms the CSS breakpoint issue.

Just a friendly reminder that this regression is still present in the latest master (B10930). Is there any ETA or blocker for PR #27985 to be merged? Thanks!

[allozaur]

Temp fix before merging #28419

[allozaur]

@benbenbenbenbenben please update e2e tests that are failing on this PR

[ServeurpersoCom]

Besides the story test, I gated the add menu reasoning submenu on single model mode, since in router mode the models selector already mounts it and it showed up twice.