diff --git a/crates/ui/src/docking/command_tests.rs b/crates/ui/src/docking/command_tests.rs
index 4f2be2f6..e109a08c 100644
--- a/crates/ui/src/docking/command_tests.rs
+++ b/crates/ui/src/docking/command_tests.rs
@@ -359,4 +359,15 @@ fn float_without_a_rect_takes_the_drag_size_inside_the_dock_area() {
     let small = FloatSpace::new(egui::Rect::from_min_size(egui::pos2(0.0, 30.0), egui::vec2(200.0, 200.0)));
     let applied = parse(json!({"command": "window.panel.float", "panel": "layers", "group": true})).unwrap().to_actions(&workspace(), false, &small);
     assert_eq!(applied, Ok(vec![Action::MoveGroup { panel: s("layers"), to: Target::Float { rect: Some([0.0, 30.0, 200.0, 200.0]) } }]));
+
+    // A host seeds the size of a panel the area never drew; a recorded size still wins.
+    let seeded = FloatSpace::new(area).with_panel_size(s("layers"), egui::vec2(180.0, 240.0));
+    let applied = parse(json!({"command": "window.panel.float", "panel": "layers"})).unwrap().to_actions(&workspace(), false, &seeded);
+    assert_eq!(applied, Ok(vec![Action::Float { panel: s("layers"), rect: [24.0, 54.0, 180.0, 240.0] }]));
+    let recorded = space.clone().with_panel_size(s("layers"), egui::vec2(180.0, 240.0));
+    assert_eq!(float_with(&recorded, "layers"), vec![Action::Float { panel: s("layers"), rect: [24.0, 54.0, 200.0, 150.0] }]);
+}
+
+fn float_with(space: &FloatSpace<P>, panel: &str) -> Vec<Action<P>> {
+    parse(json!({"command": "window.panel.float", "panel": panel})).unwrap().to_actions(&workspace(), false, space).unwrap()
 }
diff --git a/crates/ui/src/docking/surface.rs b/crates/ui/src/docking/surface.rs
index 26c92c1e..dc8f1419 100644
--- a/crates/ui/src/docking/surface.rs
+++ b/crates/ui/src/docking/surface.rs
@@ -245,6 +245,13 @@ impl<P: PartialEq> FloatSpace<P> {
     pub fn new(area: Rect) -> Self {
         Self { area, panels: Vec::new(), groups: Vec::new(), columns: Vec::new() }
     }
+
+    /// Adds the size a command floats `panel` at alone when the area has not drawn it, such as
+    /// a hidden panel or a headless run. A size the area recorded for the panel takes precedence.
+    pub fn with_panel_size(mut self, panel: P, size: Vec2) -> Self {
+        self.panels.push((panel, size));
+        self
+    }
 }
 
 /// Lookups for `DockCommand`, which needs feature `serde`.
diff --git a/docs/ui-docking.md b/docs/ui-docking.md
index 3dd46036..9705a620 100644
--- a/docs/ui-docking.md
+++ b/docs/ui-docking.md
@@ -191,7 +191,9 @@ let closed = command.apply(&mut app.workspace, style.right_to_left, &space, |pan
   inside the dock area, 24 points from its top-left corner, so it stays clear of the menu bar.
   The area records that geometry in a `FloatSpace` each frame; `float_space(ctx, id)` reads it
   and `apply` and `to_actions` take it. Before the area has drawn, `FloatSpace::new(area)` floats
-  a panel at 260x320 and a group or column at its column width by 320. `x`, `y`, `width` and
+  a panel at 260x320 and a group or column at its column width by 320.
+  `FloatSpace::with_panel_size(panel, size)` gives a panel the area has not drawn (a hidden
+  panel, a headless run) the host's own size; a size the area recorded wins. `x`, `y`, `width` and
   `height` replace the matching values, and a group or column that is already a whole floating
   window keeps its rect.
 - Without a value, `minimized`, `iconic` and `locked` toggle.