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HUD Right-Rail Transparent Strip Fix Design ​

Context ​

The HUD top toolbar is laid out in the center column while the extended right rail is a sibling in the middle horizontal flex band. When the terminal/chat panel expands, HudExtendedSidebarView currently measures itself as panel width plus rail width. The parent flex therefore reserves a full panel-width strip for the entire middle band, including the toolbar row. The expanded panel itself is bottom-anchored below the toolbar reserve, leaving the reserved area above it transparent and preventing the Sandbox toolbar from rendering there.

Prior spacing work already aligned the toolbar/control row and bottom-anchored the right-rail panels below the toolbar reserve. This design is limited to removing the remaining transparent strip caused by expanded measurement.

Decision ​

Use the rail as the only in-flow width for HudExtendedSidebarView. In expanded state the terminal/chat panel remains a child of the extended sidebar, but it is laid out as a flyout projected to the left of the rail, below the toolbar reserve. The parent HUD layout should reserve only the rail width in the middle flex band whether the panel is collapsed or expanded.

Alternatives considered:

  1. Move the HUD toolbar into the same right-dock column. This would couple independent toolbar and right-rail concerns and risks regressing recent toolbar spacing fixes.
  2. Add a special case in hud-layout to subtract the panel width when the right dock is expanded. This would leak sidebar internals into the parent layout.
  3. Measure the sidebar as rail-only and let the expanded panel project left. This matches the desired mental model, keeps the rail in flow, and preserves local ownership of panel placement in HudExtendedSidebarView.

The chosen approach is option 3.

Components and Data Flow ​

  • assets/lua/hud-extended-sidebar-view.fnl
    • Expanded measurement reports the measured rail width only.
    • Layout keeps the rail full-height at the right edge of the allocated sidebar root.
    • The active terminal/chat panel is positioned to the left of the rail and below the toolbar reserve, which may place it outside the sidebar root's measured bounds.
  • assets/lua/hud-layout.fnl
    • No behavior change is intended. It should continue reserving the right dock by the dock child's natural measured width; after the sidebar measurement change this reservation is the rail width.
  • Focused tests verify the sidebar contract directly and the HUD-layout integration behavior indirectly.

Error Handling and Constraints ​

No new runtime fallback paths are introduced. Missing required context continues to surface through existing assertions/errors. The fix must avoid legacy aliases or compatibility shims and must not change behavior outside the transparent-strip bug.

Testing Strategy ​

Follow Space Fennel UI validation order:

  1. Add/update focused Fennel tests before production changes.
  2. Run touched-file tools.fennel-check for the sidebar and layout tests.
  3. Run make constraints.
  4. Run focused HUD extended sidebar and HUD layout Fennel tests.
  5. Broaden only if the reviewer or validation evidence shows the changed surface requires it.

Acceptance criteria:

  • Expanded HudExtendedSidebarView measured width equals rail width, not rail plus panel width.
  • The expanded panel is laid out left of the rail and may project off the sidebar root while still honoring the toolbar reserve vertically.
  • HUD layout's right-dock reservation consumes only actual rail width when the dock is expanded, preserving toolbar render space.