T23e: AI chat, welcome

- WelcomeScreen: a greeting for the hour over the owner's composer,
  starters and cards, centered while it fits and scrolled from its top
  when it does not
- SuggestionChips, FollowUpSuggestions and CapabilityCards: each press asks
  the owner; rows and cards are Tab stops
- PromptLibrary is documents::TemplatePicker over saved prompts
- gallery: starters and cards fill the welcome composer, a saved prompt
  fills it too, send clears it with a toast, a follow-up asks its question

src/chat/tests/mod.rs

@@ -10,6 +10,7 @@ use gpui::{
 mod cite;
 mod compose;
 mod history;
+mod welcome;
 
 use super::{
     ImageMessage, MessageAvatar, MessageBubble, MessageEditor, MessageList, Role, ThinkingBlock,

src/chat/tests/welcome.rs

@@ -0,0 +1,55 @@
+use std::{cell::Cell, rc::Rc};
+
+use gpui::{
+    Context, IntoElement, ParentElement, Pixels, Render, Styled, TestAppContext, Window, canvas,
+    div, px,
+};
+
+use super::settle;
+use crate::{chat::WelcomeScreen, theme::Theme};
+
+/// A welcome screen of `height` over a tall body, noting where its first child sits.
+struct Welcome {
+    height: Pixels,
+    top: Rc<Cell<Pixels>>,
+}
+
+impl Render for Welcome {
+    fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
+        let top = self.top.clone();
+        div().w(px(600.0)).h(self.height).child(
+            WelcomeScreen::new("welcome", 9)
+                .child(
+                    canvas(move |bounds, _, _| top.set(bounds.top()), |_, _, _, _| {}).h(px(10.0)),
+                )
+                .child(div().h(px(400.0))),
+        )
+    }
+}
+
+#[gpui::test]
+fn the_welcome_centers_what_fits_and_scrolls_what_does_not(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let top = Rc::new(Cell::new(Pixels::ZERO));
+    let seen = top.clone();
+    let (view, cx) = cx.add_window_view(|_, _| Welcome {
+        height: px(1200.0),
+        top: seen,
+    });
+    settle(cx);
+    assert!(
+        top.get() > px(300.0),
+        "centered in a tall space, not at {:?}",
+        top.get()
+    );
+    view.update(cx, |view, cx| {
+        view.height = px(200.0);
+        cx.notify();
+    });
+    settle(cx);
+    assert!(
+        top.get() > Pixels::ZERO,
+        "a short space keeps the start in reach, not at {:?}",
+        top.get()
+    );
+}