Mainline GPUI2 UI work (#3062)

This PR mainlines the current state of new GPUI2-based UI from the
`gpui2-ui` branch.

Release Notes:

- N/A

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>
Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Nate <nate@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>

crates/ui/src/components/assistant_panel.rs

@@ -0,0 +1,91 @@
+use std::marker::PhantomData;
+
+use gpui2::geometry::rems;
+
+use crate::prelude::*;
+use crate::theme::theme;
+use crate::{Icon, IconButton, Label, Panel, PanelSide};
+
+#[derive(Element)]
+pub struct AssistantPanel<V: 'static> {
+    view_type: PhantomData<V>,
+    scroll_state: ScrollState,
+    current_side: PanelSide,
+}
+
+impl<V: 'static> AssistantPanel<V> {
+    pub fn new() -> Self {
+        Self {
+            view_type: PhantomData,
+            scroll_state: ScrollState::default(),
+            current_side: PanelSide::default(),
+        }
+    }
+
+    pub fn side(mut self, side: PanelSide) -> Self {
+        self.current_side = side;
+        self
+    }
+
+    fn render(&mut self, _: &mut V, cx: &mut ViewContext<V>) -> impl IntoElement<V> {
+        let theme = theme(cx);
+
+        struct PanelPayload {
+            pub scroll_state: ScrollState,
+        }
+
+        Panel::new(
+            self.scroll_state.clone(),
+            |_, payload| {
+                let payload = payload.downcast_ref::<PanelPayload>().unwrap();
+
+                vec![div()
+                    .flex()
+                    .flex_col()
+                    .h_full()
+                    .px_2()
+                    .gap_2()
+                    // Header
+                    .child(
+                        div()
+                            .flex()
+                            .justify_between()
+                            .gap_2()
+                            .child(
+                                div()
+                                    .flex()
+                                    .child(IconButton::new(Icon::Menu))
+                                    .child(Label::new("New Conversation")),
+                            )
+                            .child(
+                                div()
+                                    .flex()
+                                    .items_center()
+                                    .gap_px()
+                                    .child(IconButton::new(Icon::SplitMessage))
+                                    .child(IconButton::new(Icon::Quote))
+                                    .child(IconButton::new(Icon::MagicWand))
+                                    .child(IconButton::new(Icon::Plus))
+                                    .child(IconButton::new(Icon::Maximize)),
+                            ),
+                    )
+                    // Chat Body
+                    .child(
+                        div()
+                            .w_full()
+                            .flex()
+                            .flex_col()
+                            .gap_3()
+                            .overflow_y_scroll(payload.scroll_state.clone())
+                            .child(Label::new("Is this thing on?")),
+                    )
+                    .into_any()]
+            },
+            Box::new(PanelPayload {
+                scroll_state: self.scroll_state.clone(),
+            }),
+        )
+        .side(self.current_side)
+        .width(rems(32.))
+    }
+}