Add hidden-on-hover sidebar mode

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.

crates/ely_app/src/shell/chrome/appearance_layout_cards.rs

@@ -1,6 +1,8 @@
 use ely_browser_core::BrowserSnapshot;
 use ely_design_system::colors;
-use ely_domain::{COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX};
+use ely_domain::{
+    COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX, HIDDEN_SIDEBAR_WIDTH_PX,
+};
 use gpui::{
     AnyElement, Context, FontWeight, InteractiveElement, IntoElement, ParentElement, SharedString,
     StatefulInteractiveElement, Styled, div, prelude::FluentBuilder, px, rgb, rgba,
@@ -19,7 +21,7 @@ pub(crate) fn render_sidebar_layout_section(
         .find(|space| space.id() == &snapshot.active_space_id)
         .map(|space| space.sidebar_width_px())
         .unwrap_or(DEFAULT_SIDEBAR_WIDTH_PX);
-    let compact = active_width <= COLLAPSED_SIDEBAR_WIDTH_PX;
+    let mode = LayoutMode::from_width(active_width);
 
     div()
         .flex()
@@ -43,30 +45,74 @@ pub(crate) fn render_sidebar_layout_section(
         .child(
             div()
                 .grid()
-                .grid_cols(2)
+                .grid_cols(3)
                 .gap(px(10.0))
                 .child(render_layout_card(
                     "Single column",
                     "Default · workspace + tabs.",
-                    !compact,
+                    mode == LayoutMode::Single,
                     LayoutMode::Single,
                     cx,
                 ))
                 .child(render_layout_card(
                     "Compact",
                     "Icons-only with launcher rail.",
-                    compact,
+                    mode == LayoutMode::Compact,
                     LayoutMode::Compact,
                     cx,
+                ))
+                .child(render_layout_card(
+                    "Hidden on hover",
+                    "Slide in on cursor reach.",
+                    mode == LayoutMode::Hidden,
+                    LayoutMode::Hidden,
+                    cx,
                 )),
         )
         .into_any_element()
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Eq, PartialEq)]
 enum LayoutMode {
     Single,
     Compact,
+    Hidden,
+}
+
+impl LayoutMode {
+    fn from_width(width: u16) -> Self {
+        if width <= HIDDEN_SIDEBAR_WIDTH_PX {
+            Self::Hidden
+        } else if width <= COLLAPSED_SIDEBAR_WIDTH_PX {
+            Self::Compact
+        } else {
+            Self::Single
+        }
+    }
+
+    fn width(self) -> u16 {
+        match self {
+            Self::Single => DEFAULT_SIDEBAR_WIDTH_PX,
+            Self::Compact => COLLAPSED_SIDEBAR_WIDTH_PX,
+            Self::Hidden => HIDDEN_SIDEBAR_WIDTH_PX,
+        }
+    }
+
+    fn id(self) -> &'static str {
+        match self {
+            Self::Single => "layout-single",
+            Self::Compact => "layout-compact",
+            Self::Hidden => "layout-hidden",
+        }
+    }
+
+    fn preview_sidebar(self) -> f32 {
+        match self {
+            Self::Single => 56.0,
+            Self::Compact => 18.0,
+            Self::Hidden => 6.0,
+        }
+    }
 }
 
 fn render_layout_card(
@@ -76,13 +122,8 @@ fn render_layout_card(
     mode: LayoutMode,
     cx: &mut Context<ElyShell>,
 ) -> AnyElement {
-    let id = SharedString::from(match mode {
-        LayoutMode::Single => "layout-single",
-        LayoutMode::Compact => "layout-compact",
-    });
-
     div()
-        .id(id)
+        .id(SharedString::from(mode.id()))
         .p(px(14.0))
         .rounded(px(14.0))
         .bg(rgba(LAYOUT_CARD_BG))
@@ -95,11 +136,7 @@ fn render_layout_card(
         .hover(|style| style.opacity(0.94))
         .active(|style| style.opacity(0.85))
         .on_click(cx.listener(move |shell, _, _, cx| {
-            let width = match mode {
+            shell.set_active_sidebar_width(mode.width(), cx);
-                LayoutMode::Single => DEFAULT_SIDEBAR_WIDTH_PX,
-                LayoutMode::Compact => COLLAPSED_SIDEBAR_WIDTH_PX,
-            };
-            shell.set_active_sidebar_width(width, cx);
         }))
         .child(render_layout_preview(mode))
         .child(
@@ -125,10 +162,7 @@ fn render_layout_card(
 }
 
 fn render_layout_preview(mode: LayoutMode) -> AnyElement {
-    let sidebar_width = match mode {
+    let sidebar_width = mode.preview_sidebar();
-        LayoutMode::Single => 56.0,
-        LayoutMode::Compact => 18.0,
-    };
 
     div()
         .h(px(96.0))

crates/ely_app/src/shell/mod.rs

@@ -66,6 +66,7 @@ pub struct ElyShell {
     pub(crate) plugin_search_input: Entity<InputState>,
     pub(crate) translucency_slider: Entity<SliderState>,
     pub(crate) workspace_picker_open: bool,
+    pub(crate) sidebar_hover_expanded: bool,
     download_action_error: Option<String>,
     download_clear_confirmation: bool,
     download_security_confirmation: Option<PendingDownloadFileAction>,
@@ -177,6 +178,7 @@ impl ElyShell {
             plugin_search_input,
             translucency_slider,
             workspace_picker_open: false,
+            sidebar_hover_expanded: false,
             download_action_error: None,
             download_clear_confirmation: false,
             download_security_confirmation: None,

crates/ely_app/src/shell/render.rs

@@ -1,9 +1,9 @@
 use ely_browser_core::BrowserSnapshot;
 use ely_design_system::{colors, spacing};
-use ely_domain::BrowserTab;
+use ely_domain::{BrowserTab, DEFAULT_SIDEBAR_WIDTH_PX, HIDDEN_SIDEBAR_WIDTH_PX};
 use gpui::{
-    AnyElement, Context, InteractiveElement, IntoElement, ParentElement, Render, Styled, Window,
+    AnyElement, Context, InteractiveElement, IntoElement, ParentElement, Render, SharedString,
-    div, px, rgb, rgba,
+    StatefulInteractiveElement, Styled, Window, div, prelude::FluentBuilder, px, rgb, rgba,
 };
 
 use super::chrome::{
@@ -36,7 +36,9 @@ impl ElyShell {
             Ok(sidebar_width) => sidebar_width,
             Err(message) => return render_error(message),
         };
-        let sidebar_collapsed = collapsed_sidebar_active(sidebar_width);
+        let sidebar_hidden = sidebar_width <= f32::from(HIDDEN_SIDEBAR_WIDTH_PX);
+        let sidebar_collapsed = collapsed_sidebar_active(sidebar_width) && !sidebar_hidden;
+        let hover_expanded = sidebar_hidden && self.sidebar_hover_expanded;
 
         div()
             .size_full()
@@ -72,13 +74,52 @@ impl ElyShell {
                     .p(px(spacing::SHELL_INSET))
                     .gap(px(spacing::SIDEBAR_MAIN_GAP))
                     .flex()
-                    .child(self.render_sidebar(&snapshot, sidebar_width, sidebar_collapsed, cx))
+                    .child(self.render_sidebar(
+                        &snapshot,
+                        sidebar_width,
+                        sidebar_collapsed,
+                        sidebar_hidden,
+                        cx,
+                    ))
                     .child(self.render_main_pane(&snapshot, &active_tab, sidebar_collapsed, cx)),
             )
+            .when(hover_expanded, |el| {
+                el.child(self.render_hidden_sidebar_overlay(&snapshot, cx))
+            })
             .children(render_command_overlay(&snapshot, cx))
             .into_any_element()
     }
 
+    fn render_hidden_sidebar_overlay(
+        &mut self,
+        snapshot: &BrowserSnapshot,
+        cx: &mut Context<Self>,
+    ) -> AnyElement {
+        div()
+            .absolute()
+            .inset_0()
+            .child(
+                div()
+                    .id(SharedString::from("hidden-sidebar-backdrop"))
+                    .absolute()
+                    .inset_0()
+                    .on_click(cx.listener(|shell, _, _, cx| shell.collapse_hidden_sidebar(cx))),
+            )
+            .child(
+                div()
+                    .absolute()
+                    .top(px(spacing::SHELL_INSET))
+                    .left(px(spacing::SHELL_INSET))
+                    .bottom(px(spacing::SHELL_INSET))
+                    .child(self.render_expanded_sidebar(
+                        snapshot,
+                        f32::from(DEFAULT_SIDEBAR_WIDTH_PX),
+                        cx,
+                    )),
+            )
+            .into_any_element()
+    }
+
     fn render_main_pane(
         &mut self,
         snapshot: &BrowserSnapshot,
@@ -112,13 +153,31 @@ impl ElyShell {
         snapshot: &BrowserSnapshot,
         sidebar_width: f32,
         sidebar_collapsed: bool,
+        sidebar_hidden: bool,
         cx: &mut Context<Self>,
     ) -> AnyElement {
+        if sidebar_hidden {
+            return self.render_hidden_sidebar_rail(cx);
+        }
         if sidebar_collapsed {
             return self.render_compact_sidebar(snapshot, sidebar_width, cx);
         }
         self.render_expanded_sidebar(snapshot, sidebar_width, cx)
     }
+
+    fn render_hidden_sidebar_rail(&mut self, cx: &mut Context<Self>) -> AnyElement {
+        div()
+            .id(SharedString::from("hidden-sidebar-rail"))
+            .w(px(f32::from(HIDDEN_SIDEBAR_WIDTH_PX)))
+            .h_full()
+            .rounded(px(spacing::RADIUS_PANE))
+            .bg(rgba(0xffffff8c))
+            .cursor_pointer()
+            .hover(|style| style.bg(rgba(0xffffffd9)))
+            .active(|style| style.opacity(0.78))
+            .on_click(cx.listener(|shell, _, _, cx| shell.expand_hidden_sidebar(cx)))
+            .into_any_element()
+    }
 }
 
 fn render_error(message: String) -> AnyElement {

crates/ely_app/src/shell/sidebar.rs

@@ -1,7 +1,8 @@
 use ely_browser_core::BrowserSnapshot;
 use ely_design_system::{colors, spacing};
 use ely_domain::{
-    ArchivedTab, BrowserTab, COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX, Space,
+    ArchivedTab, BrowserTab, COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX,
+    HIDDEN_SIDEBAR_WIDTH_PX, Space,
 };
 use gpui::{
     AnyElement, BoxShadow, Context, IntoElement, ParentElement, Styled, Window, div, hsla, point,
@@ -124,6 +125,23 @@ impl ElyShell {
         };
 
         if core.set_space_sidebar_width(&active_space_id, width_px).is_ok() {
+            if width_px > HIDDEN_SIDEBAR_WIDTH_PX {
+                self.sidebar_hover_expanded = false;
+            }
+            cx.notify();
+        }
+    }
+
+    pub(crate) fn expand_hidden_sidebar(&mut self, cx: &mut Context<Self>) {
+        if !self.sidebar_hover_expanded {
+            self.sidebar_hover_expanded = true;
+            cx.notify();
+        }
+    }
+
+    pub(crate) fn collapse_hidden_sidebar(&mut self, cx: &mut Context<Self>) {
+        if self.sidebar_hover_expanded {
+            self.sidebar_hover_expanded = false;
             cx.notify();
         }
     }

crates/ely_domain/src/lib.rs

@@ -58,7 +58,10 @@ pub use site_permission::{
     SiteOrigin, SitePermissionAuditAction, SitePermissionAuditEvent, SitePermissionDecision,
     SitePermissionEntry, SitePermissionFeature,
 };
-pub use space::{ArchivePolicy, COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX, Space};
+pub use space::{
+    ArchivePolicy, COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX, HIDDEN_SIDEBAR_WIDTH_PX,
+    Space,
+};
 pub use split::{MAX_SPLIT_PANES, SplitAxis, SplitLayout, SplitPane};
 pub use sync::{
     SyncConnectionState, SyncObjectKind, SyncObjectPolicy, SyncObjectState, SyncObjectStatus,

crates/ely_domain/src/space.rs

@@ -4,6 +4,7 @@ use crate::{ProfileId, SpaceId};
 
 pub const DEFAULT_SIDEBAR_WIDTH_PX: u16 = 280;
 pub const COLLAPSED_SIDEBAR_WIDTH_PX: u16 = 56;
+pub const HIDDEN_SIDEBAR_WIDTH_PX: u16 = 8;
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum ArchivePolicy {