slint-ui/slint · error

no simulated native drag in flight

Error message

no simulated native drag in flight

What it means

The testing backend's `deliver_native_drag` requires a simulated native drag to have been started beforehand; it reads drag data from an internal `native_drag` cell. If no drag is in flight the cell is `None` and the backend panics.

Solutions

1. Start the simulated native drag (drag-enter/press with drag data) before calling move or drop
2. Ensure each drop has exactly one preceding drag start; a second drop after the drag was consumed also panics
3. Reset or recreate the test backend/window between drag scenarios

Example fix

// before
backend.simulate_native_drop(&window, pos); // panics: no drag in flight
// after
backend.simulate_native_drag_enter(&window, drag_data);
backend.simulate_native_drag_move(&window, pos);
backend.simulate_native_drop(&window, pos);

// ensure a drag was started before move/drop in your test harness
fn ensure_drag_in_flight(backend: &TestingBackend, started: &mut bool) {
    assert!(*started, "call drag-enter/start before move or drop");
}

std::panic::catch_unwind(|| backend.simulate_native_drop(&window, pos))
    .map_err(|_| anyhow!("no native drag in flight: start drag first"))?;

When it happens

Trigger: Calling `simulate_native_drag_move()` or `simulate_native_drop()` without a prior call that starts a simulated native drag (e.g. `simulate_native_drag_enter`/equivalent start API) on the testing backend.

Common situations: Test scripts that skip the drag-start step; reordering refactored test steps so drop/move happens after the drag was already consumed by a previous drop; writing only a 'drop' simulation assuming it stands alone.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• a setter belongs to a field
• an identifier
• binding was of the wrong type
• C++: NativeWindowHandle created with null hwnd
• C++: NativeWindowHandle created with null xcb window handle

AI-assisted analysis of slint-ui/slint@bb937076de (2026-09-16). Data as JSON: /api/errors/4088cd629ec3926f. Report an issue: GitHub.

Appendix: source

Thrown at internal/backends/testing/testing_backend.rs:416

    /// the final negotiated action.
    pub fn simulate_native_drop(
        &self,
        target: &i_slint_core::api::Window,
        position: LogicalPosition,
    ) -> DragAction {
        let action = self.deliver_native_drag(target, position, true);
        WindowInner::from_pub(&self.window).report_drag_finished(action);
        action
    }

    fn deliver_native_drag(
        &self,
        target: &i_slint_core::api::Window,
        position: LogicalPosition,
        drop: bool,
    ) -> DragAction {
        let (data, allowed) =
            self.native_drag.borrow().clone().expect("no simulated native drag in flight");
        let mut event = DropEvent::default();
        event.data = data;
        event.position = position;
        event.proposed_action =
            i_slint_core::items::compute_proposed_action(Default::default(), allowed);
        let event = if drop {
            BackendDragEvent::Drop { event, allowed }
        } else {
            BackendDragEvent::Move { event, allowed }
        };
        WindowInner::from_pub(target).process_drag_event(event).unwrap_or(DragAction::None)
    }
}

impl WindowAdapterInternal for TestingWindow {
    fn input_method_request(&self, request: i_slint_core::window::InputMethodRequest) {
        self.ime_requests.borrow_mut().push(request)
    }

View on GitHub (pinned to bb937076de)