Sidebar missing on first launch

[ryan-carpenter]

Issue Description

On first launch, Cinny opened showing the Welcome page with "Source Code" and "Support" buttons, but no visible sidebar, even though the window was wide enough to show the sidebar.

Expected Behavior

I expected the to see the sidebar by default when the window is wide enough to accommodate it. When the sidebar was not visible, despite the window being wide enough, I expected to find a graphical control to reveal the sidebar.

Actual Behavior

The sidebar did not appear until after I opened the app window in full screen. After this, the sidebar behaved normally, meaning it remained visible at any window size, except at narrow widths.

Reproduction Steps

1. Install Cinny Desktop
2. Launch for the first time --> Sidebar is not visible
3. Open Cinny in fullscreen --> Sidebar appears
4. Restore window size --> Sidebar remains visible
5. Quit and relaunch Cinny Desktop --> Sidebar remains visible

Cinny Desktop was positioned (possibly tiled) on the left side of my screen, meaning I must have moved it from the initial position. Still, the sidebar was not visible until I opened in full screen.

Environment

- OS: macOS Tahoe
- Browser: Firefox 152.0.6
- Cinny desktop version: 4.12.2 from nixpkgs or 4.12.5 from GitHub (`.dmg` or `.app.tar.gz`)
- Matrix Homeserver: matrix.org

Relevant Logs

No response

Additional context

This happened after installing from nixpkgs, so I uninstalled and tried with the current release from GitHub, getting the same result. The sidebar only appeared after I opened Cinny in fullscreen. Realizing this might not be a broken package, I reinstalled from nixpkgs and now Cinny opened with the sidebar showing and the same window size and position as last used.

Claude.ai guesses that this is the explanation:

The sidebar's visibility is likely gated by a layout check (CSS media query or JS resize listener) that runs once at initial render, using window dimensions that aren't yet finalized by the OS/WebView. A manual drag-resize doesn't fire the same event type the layout check listens for, but a fullscreen toggle does — triggering a one-time re-evaluation that then sticks because the sidebar's visibility state is retained rather than re-checked afterward.

I acknowledge that:

• I have searched the Cinny repository (both open and closed Discussions and Issues) and confirm this is not a duplicate of an existing issue or discussion.
• I have checked the "Preview" tab on all text fields to ensure that everything looks right, and have wrapped all configuration and code in code blocks with a group of three backticks (```) on separate lines.