feat: out-of-box experience (#2)

Reviewed-on: https://git.reya.su/reya/signed/pulls/2

crates/dock/src/invalid_panel.rs

@@ -0,0 +1,71 @@
+use gpui::{
+    App, Context, EventEmitter, FocusHandle, Focusable, IntoElement, ParentElement as _, Render,
+    SharedString, Styled as _, Window, div,
+};
+use gpui_base::dock::{PanelEvent, PanelState};
+use gpui_component::ActiveTheme as _;
+
+use crate::Panel;
+
+/// Stands in for a panel this build cannot construct — one whose `panel_name`
+/// no [`PanelRegistry`](gpui_base::dock::PanelRegistry) builder answers to.
+///
+/// It reports the original [`PanelState`] from
+/// [`dump`](gpui_base::dock::Panel::dump), so a layout written by a build that
+/// knows the panel survives a load and a save here rather than losing it.
+pub(crate) struct InvalidPanel {
+    name: SharedString,
+    focus_handle: FocusHandle,
+    old_state: PanelState,
+}
+
+impl InvalidPanel {
+    pub(crate) fn new(
+        name: impl Into<SharedString>,
+        state: PanelState,
+        cx: &mut Context<Self>,
+    ) -> Self {
+        Self {
+            focus_handle: cx.focus_handle(),
+            name: name.into(),
+            old_state: state,
+        }
+    }
+}
+
+impl gpui_base::dock::Panel for InvalidPanel {
+    fn panel_name(&self) -> &'static str {
+        "InvalidPanel"
+    }
+
+    fn dump(&self, _: &App) -> PanelState {
+        self.old_state.clone()
+    }
+}
+
+impl Panel for InvalidPanel {}
+
+impl EventEmitter<PanelEvent> for InvalidPanel {}
+
+impl Focusable for InvalidPanel {
+    fn focus_handle(&self, _: &App) -> FocusHandle {
+        self.focus_handle.clone()
+    }
+}
+
+impl Render for InvalidPanel {
+    fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        div()
+            .size_full()
+            .my_6()
+            .flex()
+            .flex_col()
+            .items_center()
+            .justify_center()
+            .text_color(cx.theme().muted_foreground)
+            .child(format!(
+                "The `{}` panel type is not registered in PanelRegistry.",
+                self.name.clone()
+            ))
+    }
+}