Dock: reorder tabs by dragging, one width per tab group, plain rail hover

- Dragging a tab along its slot's tab strip moves it to the gap under the
  pointer; the drop preview shows the new tab order. A panel dragged from
  elsewhere onto a tab strip lands at that position too. A drop in the
  middle of the slot still appends a newcomer and leaves a member where
  it is.
- All tabs of a slot share one width: a joining panel takes the slot's
  width, resizing or resetting one tab sizes them all, and slots sized
  apart by older configs render at their widest tab. Switching tabs no
  longer changes the column width.
- Hovering an auto-hide rail raises its slot without turning the rail
  blue.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013RyGiMXcsf9HTKfqDxaQGt

src/app/view/mod.rs

@@ -2773,7 +2773,7 @@ impl OpenCADStudio {
         // column is as wide as the widest one currently showing.
         let col_w = expanded
             .iter()
-            .map(|gi| self.dock.width(shown(*gi), self.win_size.0))
+            .map(|gi| self.dock_group_width(side, *gi))
             .fold(0.0, f32::max);
 
         let tab_strip = (!pinned.is_empty()).then(|| {
@@ -2785,7 +2785,6 @@ impl OpenCADStudio {
                         shown(*gi),
                         &tabs,
                         side,
-                        self.dock_expanded.is_some_and(|e| tabs.contains(&e)),
                         self.dock_dragging.is_some_and(|d| tabs.contains(&d)),
                     )
                 })
@@ -2844,7 +2843,6 @@ impl OpenCADStudio {
         id: crate::ui::dock::PanelId,
         tabs: &[crate::ui::dock::PanelId],
         side: crate::app::config::DockSide,
-        is_active: bool,
         is_dragging: bool,
     ) -> Element<'_, Message> {
         let label = canvas(VBarLabel {
@@ -2858,11 +2856,10 @@ impl OpenCADStudio {
         })
         .width(Fill)
         .height(Fill);
+        // Hovering raises the slot but leaves the rail looking as it is.
         let bg = move |theme: &Theme| {
             let palette = theme.palette();
-            if is_active {
-                palette.primary.weak.color
-            } else if is_dragging {
+            if is_dragging {
                 palette.primary.weak.color.scale_alpha(0.55)
             } else {
                 palette.background.base.color
@@ -2874,11 +2871,7 @@ impl OpenCADStudio {
             .style(move |theme: &Theme| container::Style {
                 background: Some(Background::Color(bg(theme))),
                 border: Border {
-                    color: if is_active {
-                        theme.palette().primary.base.color
-                    } else {
-                        theme.palette().background.neutral.color
-                    },
+                    color: theme.palette().background.neutral.color,
                     width: 1.0,
                     radius: 0.0.into(),
                 },
@@ -3117,16 +3110,23 @@ impl OpenCADStudio {
                     top,
                 ));
             }
-            DropTarget::Tab { side, group } => {
+            DropTarget::Tab { side, group, index } => {
                 let (top, bottom) = self
                     .dock_slot_spans(side)
                     .into_iter()
                     .find(|(gi, _, _)| *gi == group)
                     .map(|(_, t, b)| (t, b))
                     .unwrap_or((0.0, 0.0));
-                let mut tabs = self.dock_group_visible_tabs(side, group);
-                tabs.retain(|t| *t != id);
-                tabs.push(id);
+                // The slot's tabs as they will be after the drop.
+                let mut preview = self.dock.clone();
+                preview.add_tab(id, side, group, index);
+                let tabs: Vec<_> = preview
+                    .location(id)
+                    .map(|(s, gi)| preview.groups(s)[gi].tabs.clone())
+                    .unwrap_or_default()
+                    .into_iter()
+                    .filter(|t| *t == id || self.dock_panel_visible(*t))
+                    .collect();
                 let w = edge_w(side);
                 layers.push(tint(side));
                 layers.push(place_at(
@@ -3576,10 +3576,6 @@ impl canvas::Program<Message> for VBarLabel {
     }
 }
 
-/// Width of a collapsed (auto-collapsing) panel's tab in the edge strip.
-const DOCK_RAIL_W: f32 = 28.0;
-/// Width of the grabbable divider between a docked panel and the viewport.
-const DOCK_DIVIDER_W: f32 = 5.0;
 /// Width of a floating panel's vertical title bar.
 const DOCK_FLOAT_BAR_W: f32 = 28.0;
 
@@ -3645,8 +3641,7 @@ fn dock_divider(id: crate::ui::dock::PanelId) -> Element<'static, Message> {
         .into()
 }
 
-/// Height of a docked slot's tab strip.
-const DOCK_TAB_H: f32 = 24.0;
+use crate::ui::dock::{DOCK_DIVIDER_W, DOCK_RAIL_W, DOCK_TAB_H};
 
 /// The tab row above a slot holding several panels. Pressing a tab shows its
 /// panel; dragging it pulls the panel out of the slot.