LanMountainDesktop/.trae/specs/settings-window-fluent-shell-redesign/spec.md

Settings Window Fluent Shell Redesign

Goal

Rebuild the settings window as an independent Fluent shell with a custom titlebar, titlebar hamburger menu, persistent side navigation, search, and Avalonia-standard system material support.

Requirements

• Keep the existing independent settings-window lifecycle: open-or-focus, no owner anchor, own taskbar entry.
• Use a 48 DIP titlebar with Back, pane toggle, icon/title, search, restart action, more menu, and caption-button spacer.
• Keep the titlebar and content area on one shared full-window background layer; the custom titlebar must remain transparent and must not paint a contrasting strip.
• Avoid a visible titlebar bottom divider that makes the titlebar read as a separate color band.
• Keep FANavigationView as the primary navigation surface with OpenPaneLength around 283 DIP.
• Keep FANavigationView pane and content template backgrounds transparent in the settings shell so the navigation control does not reintroduce a second surface color.
• Move the compact/minimal pane toggle from the navigation footer into the titlebar.
• Add search over built-in settings pages and settings expanders; selecting a result navigates, expands, focuses, and highlights.
• Add auto system material mode and make it the default.
• Implement material with Avalonia TransparencyLevelHint only.
• Preserve settings page layout as direct ScrollViewer -> StackPanel -> FASettingsExpander content.
• Follow docs/VISUAL_SPEC.md, docs/CORNER_RADIUS_SPEC.md, and docs/ai/SETTINGS_WINDOW_DESIGN.md.

Acceptance

• dotnet build LanMountainDesktop.slnx -c Debug succeeds.
• dotnet test LanMountainDesktop.slnx -c Debug succeeds or any unrelated failures are documented.
• The settings window can navigate by sidebar, titlebar Back, titlebar pane toggle, and search.
• Appearance settings expose Auto, None, Mica, and/or Acrylic according to system support.
• Existing dirty user changes are not reverted.