diff --git a/crates/ui-egui/src/panel_docking.rs b/crates/ui-egui/src/panel_docking.rs
index d01858d3c..7e1df99c4 100644
--- a/crates/ui-egui/src/panel_docking.rs
+++ b/crates/ui-egui/src/panel_docking.rs
@@ -625,6 +625,17 @@ pub(crate) fn show(app: &mut VectorcraftApp, ui: &mut egui::Ui) -> bool {
     display
         .floating
         .retain(|group| !app.ui.floating_panels.iter().any(|native| decorated(native) && native.panels.iter().any(|id| group.panels.contains(id))));
+    // Floating groups lie on the canvas, not on the dock's card: reserve a fill under each one
+    // before the dock draws it, sized once it is placed, so the canvas doesn't show through (#2421).
+    let fills: Vec<_> = display
+        .floating
+        .iter()
+        .filter_map(|group| group.panels.first())
+        .map(|first| {
+            let layer = egui::LayerId::new(egui::Order::Middle, area.with(("floating", first)));
+            (layer, ui.ctx().layer_painter(layer).add(egui::Shape::Noop))
+        })
+        .collect();
     let mut draw = |ui: &mut egui::Ui| {
         if layout.root.is_some() && crate::dock::collapse_header(ui, collapsed) {
             let _ = app.run("window.collapseDock", json!({"collapsed":!collapsed}));
@@ -644,7 +655,8 @@ pub(crate) fn show(app: &mut VectorcraftApp, ui: &mut egui::Ui) -> bool {
         }
         let mut style = craft_ui::docking::DockStyle::from_ui(ui);
         style.tab_height = 33.0;
-        // The dock is a card (`theme::card`): its panes and tab strip lie on the card's colour.
+        // The dock is a card (`theme::card`): its panes and tab strip lie on the card's colour
+        // (square fills would cover its rounded corners). Floating groups get the fills below.
         style.background = egui::Color32::TRANSPARENT;
         style.tab_background = egui::Color32::TRANSPARENT;
         style.active_background = t.button;
@@ -659,7 +671,7 @@ pub(crate) fn show(app: &mut VectorcraftApp, ui: &mut egui::Ui) -> bool {
         style.resize_label = tl!("Resize panels").to_string();
         style.resize_window_label = tl!("Resize panel window").to_string();
         let mut content = PanelContent { app, commands: Vec::new() };
-        let output = craft_ui::docking::DockArea::new(egui::Id::new("vectorcraft-panel-docking")).show_customized(
+        let output = craft_ui::docking::DockArea::new(area).show_customized(
             ui,
             &display,
             &style,
@@ -754,6 +766,11 @@ pub(crate) fn show(app: &mut VectorcraftApp, ui: &mut egui::Ui) -> bool {
             ui.ctx().data_mut(|data| data.insert_temp(crate::floating::group_rect_id(first), rect));
         }
     }
+    for (layer, fill) in fills {
+        if let Some(rect) = ui.ctx().memory(|memory| memory.area_rect(layer.id)) {
+            ui.ctx().layer_painter(layer).set(fill, egui::Shape::rect_filled(rect, 0.0, t.panel));
+        }
+    }
     icon_rail(app, ui);
     crate::floating::docked_columns(app, ui);
     true
diff --git a/crates/ui-egui/src/panel_docking_tests.rs b/crates/ui-egui/src/panel_docking_tests.rs
index c66623e75..f0a556d51 100644
--- a/crates/ui-egui/src/panel_docking_tests.rs
+++ b/crates/ui-egui/src/panel_docking_tests.rs
@@ -1,5 +1,5 @@
 use super::*;
-use egui::{Event, Modifiers, PointerButton, Pos2, vec2};
+use egui::{Event, Modifiers, PointerButton, Pos2, pos2, vec2};
 use egui_kittest::{Harness, kittest::Queryable};
 
 fn app() -> VectorcraftApp {
@@ -614,3 +614,29 @@ fn the_window_menu_item_hides_a_shown_panel_and_shows_it_again() {
     app.run("window.panel", json!({"panel":"properties"})).unwrap();
     assert!(shown(&app), "without the menu's toggle the command only shows the panel");
 }
+
+/// A floating group lies on the canvas, not on the dock's card, so its tab strip and body paint
+/// their own opaque background (#2421).
+#[test]
+fn floating_group_paints_an_opaque_background() {
+    let mut app = app();
+    app.run("window.panel.float", json!({"panel":"properties","x":400,"y":200,"width":300,"height":420})).unwrap();
+    let h = harness(app, vec2(1280.0, 800.0));
+    // Paint order: the last big filled rectangle over a point decides what the user sees there
+    // (outlines, small widgets and fully transparent fills, which paint nothing, don't count).
+    fn topmost(shape: &egui::Shape, p: Pos2, found: &mut Option<egui::Color32>) {
+        match shape {
+            egui::Shape::Rect(r) if r.rect.contains(p) && r.rect.width() >= 200.0 && r.stroke.is_empty() && r.fill.a() > 0 => *found = Some(r.fill),
+            egui::Shape::Vec(v) => v.iter().for_each(|s| topmost(s, p, found)),
+            _ => {}
+        }
+    }
+    let opaque_at = |p: Pos2| {
+        let mut found = None;
+        h.output().shapes.iter().for_each(|s| topmost(&s.shape, p, &mut found));
+        found.is_some_and(|c| c.a() == 255)
+    };
+    // Tab strip to the right of the tabs, and the body's empty bottom-right corner.
+    assert!(opaque_at(pos2(690.0, 210.0)), "tab strip is transparent");
+    assert!(opaque_at(pos2(690.0, 610.0)), "body is transparent");
+}