Domain ships HIDDEN_SIDEBAR_WIDTH_PX = 8 alongside the existing
collapsed/default tiers. ElyShell tracks sidebar_hover_expanded with
expand_hidden_sidebar / collapse_hidden_sidebar helpers; switching
back to a non-hidden width via the layout cards or core API resets
the flag automatically so the sidebar can never be both hidden and
expanded after a mode change.
Renderer:
- render_sidebar takes a sidebar_hidden flag and routes to a thin
8 px clickable rail (hover bg + click expands) when the active
space's width is at HIDDEN.
- While the rail is expanded, render_browser overlays a transparent
backdrop + the full default-width sidebar absolutely positioned in
the shell inset, so the main pane content never reflows.
- collapsed_sidebar_active still drives the COLLAPSED-tier compact
sidebar; the hidden tier is opted out of that path.
Appearance form:
- Layout cards section gains the design's third "Hidden on hover"
card with a 6 px sliver preview that mutates the active space to
HIDDEN_SIDEBAR_WIDTH_PX. LayoutMode now derives id/width/preview
from a small enum so adding a fourth mode would be one match arm.
The hover-expanded state never persists into the domain; once the
user switches modes or clicks the backdrop, it collapses cleanly.
ZacharyZhang-NY