Fullscreen window layout can obscure the text input and working area on Windows

When the ChatGPT / Codex desktop app is used in fullscreen mode on Windows, switching between areas or showing the system/navigation bar can obscure part of the working area, including the text input field.

This makes the app difficult to use during active work, because the user has to move between chat, previews, side panels, and input controls while parts of the interface are hidden or covered.

Expected behavior: fullscreen mode should preserve a safe visible area for the text input, chat messages, previews, and main controls. If a bar or overlay appears, the layout should adapt so it does not cover essential UI elements.

Steps to reproduce:

1. Open the ChatGPT / Codex desktop app on Windows.
2. Put the app in fullscreen mode.
3. Work with chat, previews, or side panels open.
4. Move between areas or trigger the system/navigation bar.
5. Observe that part of the app interface, including the text input area, can be obscured.

Expected: the fullscreen layout should avoid covering the input field and main working areas.

Additional information: this is a separate issue from the non-draggable voice overlay bug, although both affect the usability of the app during real project work.

Start by reproducing the issue in the Windows desktop app with fullscreen mode, chat, previews, and side panels open, then trigger the system/navigation bar. Trace the fullscreen layout entry point and verify that the text input, chat messages, previews, and main controls remain visible when bars or overlays appear.