diff --git a/crates/ui-egui/src/panel_docking.rs b/crates/ui-egui/src/panel_docking.rs
index d01858d3c..9cb1cdbf6 100644
--- a/crates/ui-egui/src/panel_docking.rs
+++ b/crates/ui-egui/src/panel_docking.rs
@@ -644,9 +644,11 @@ 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.
-        style.background = egui::Color32::TRANSPARENT;
-        style.tab_background = egui::Color32::TRANSPARENT;
+        // The dock is a card (`theme::card`) filled with the panel colour. Floating groups lie on
+        // the canvas, not on that card, so panes and tab strip paint the same colour themselves;
+        // transparent they showed the canvas through (#2421).
+        style.background = t.panel;
+        style.tab_background = t.panel;
         style.active_background = t.button;
         style.text = t.text_strong;
         style.inactive_text = t.text_dim;
diff --git a/crates/ui-egui/src/panel_docking_tests.rs b/crates/ui-egui/src/panel_docking_tests.rs
index c66623e75..9eccbf0e8 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 and small widgets 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() => *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");
+}