Widget changed layer_id during the frame panic when dragging tab into floating window (only debug builds)

[mcthesw]

Describe the bug
In debug builds, dragging a tab out of the main dock to create a floating window panics with:

thread 'main' panicked at egui-0.34.x/src/widget_rect.rs:NNN:
DEBUG ASSERT: Widget XXXX changed layer_id during the frame from LayerId { Background ... } to LayerId { Middle ... }

To Reproduce
requires egui_dock with debug assertions enabled

// [dependencies]
// egui = "0.34"
// egui_dock = "0.19"

use egui::{Id, Pos2, RawInput, Rect, Vec2};
use egui_dock::{DockArea, DockState, NodeIndex, SurfaceIndex, TabIndex, TabPath, TabViewer};

struct Tab(u8);
struct Viewer;

impl TabViewer for Viewer {
    type Tab = Tab;

    fn title(&mut self, tab: &mut Self::Tab) -> egui::WidgetText {
        format!("Tab {}", tab.0).into()
    }

    // Both tabs return the same id — the default impl derives from title(),
    // so any two tabs with the same title will also reproduce this.
    fn id(&mut self, _tab: &mut Self::Tab) -> Id {
        Id::new("same-tab-id")
    }

    fn ui(&mut self, ui: &mut egui::Ui, tab: &mut Self::Tab) {
        ui.label(format!("content {}", tab.0));
    }
}

fn main() {
    let ctx = egui::Context::default();
    let mut dock_state = DockState::new(vec![Tab(0), Tab(1)]);

    dock_state.detach_tab(
        TabPath {
            surface: SurfaceIndex::main(),
            node: NodeIndex::root(),
            tab: TabIndex(0),
        },
        Rect::from_min_size(Pos2::new(50.0, 50.0), Vec2::new(200.0, 150.0)),
    );

    let mut input = RawInput::default();
    input.screen_rect = Some(Rect::from_min_size(Pos2::ZERO, Vec2::new(800.0, 600.0)));

    // Panics in debug builds:
    //   Widget XXXX changed layer_id from LayerId { Background ... } to LayerId { Middle ... }
    let _ = ctx.run_ui(input, |ui| {
        DockArea::new(&mut dock_state).show_inside(ui, &mut Viewer);
    });
}

Expected behavior
No panic.

Additional context
The panic originates in tab_body in leaf.rs. The tab body Ui is constructed with:

let id = self.id.with(tab_viewer.id(tab));

This ID doesn't include the surface index, so if the main dock surface and a detached floating window surface both render a tab with the same TabViewer::id in the same frame (which happens during/after an undock), egui sees the same widget ID registered in two different layers (Background for the dock, Middle for the window) and fires the assertion.