assistant2: Add scrollbar to active thread (#27534)

This required adding scrollbar support to `list`. Since `list` is
virtualized, the scrollbar height will change as more items are
measured. When the user manually drags the scrollbar, we'll persist the
initial height and offset calculations accordingly to prevent the
scrollbar from moving away from the cursor as new items are measured.

We're not doing this yet, but in the future, it'd be nice to budget some
time each frame to layout unmeasured items so that the scrollbar height
is as accurate as possible.

Release Notes:

- N/A

crates/assistant2/src/active_thread.rs

@@ -6,16 +6,15 @@ use crate::thread_store::ThreadStore;
 use crate::tool_use::{PendingToolUseStatus, ToolUse, ToolUseStatus};
 use crate::ui::{ContextPill, ToolReadyPopUp, ToolReadyPopupEvent};
 use crate::AssistantPanel;
-
 use assistant_settings::AssistantSettings;
 use collections::HashMap;
 use editor::{Editor, MultiBuffer};
 use gpui::{
     linear_color_stop, linear_gradient, list, percentage, pulsating_between, AbsoluteLength,
     Animation, AnimationExt, AnyElement, App, ClickEvent, DefiniteLength, EdgesRefinement, Empty,
-    Entity, Focusable, Hsla, Length, ListAlignment, ListOffset, ListState, ScrollHandle,
-    StyleRefinement, Subscription, Task, TextStyleRefinement, Transformation, UnderlineStyle,
-    WeakEntity, WindowHandle,
+    Entity, Focusable, Hsla, Length, ListAlignment, ListOffset, ListState, MouseButton,
+    ScrollHandle, Stateful, StyleRefinement, Subscription, Task, TextStyleRefinement,
+    Transformation, UnderlineStyle, WeakEntity, WindowHandle,
 };
 use language::{Buffer, LanguageRegistry};
 use language_model::{LanguageModelRegistry, LanguageModelToolUseId, Role};
@@ -24,7 +23,7 @@ use settings::Settings as _;
 use std::sync::Arc;
 use std::time::Duration;
 use theme::ThemeSettings;
-use ui::{prelude::*, Disclosure, IconButton, KeyBinding, Tooltip};
+use ui::{prelude::*, Disclosure, IconButton, KeyBinding, Scrollbar, ScrollbarState, Tooltip};
 use util::ResultExt as _;
 use workspace::{OpenOptions, Workspace};
 
@@ -39,6 +38,7 @@ pub struct ActiveThread {
     save_thread_task: Option<Task<()>>,
     messages: Vec<MessageId>,
     list_state: ListState,
+    scrollbar_state: ScrollbarState,
     rendered_messages_by_id: HashMap<MessageId, RenderedMessage>,
     rendered_tool_use_labels: HashMap<LanguageModelToolUseId, Entity<Markdown>>,
     editing_message: Option<(MessageId, EditMessageState)>,
@@ -227,6 +227,14 @@ impl ActiveThread {
             cx.subscribe_in(&thread, window, Self::handle_thread_event),
         ];
 
+        let list_state = ListState::new(0, ListAlignment::Bottom, px(2048.), {
+            let this = cx.entity().downgrade();
+            move |ix, window: &mut Window, cx: &mut App| {
+                this.update(cx, |this, cx| this.render_message(ix, window, cx))
+                    .unwrap()
+            }
+        });
+
         let mut this = Self {
             language_registry,
             thread_store,
@@ -239,13 +247,8 @@ impl ActiveThread {
             rendered_tool_use_labels: HashMap::default(),
             expanded_tool_uses: HashMap::default(),
             expanded_thinking_segments: HashMap::default(),
-            list_state: ListState::new(0, ListAlignment::Bottom, px(1024.), {
-                let this = cx.entity().downgrade();
-                move |ix, window: &mut Window, cx: &mut App| {
-                    this.update(cx, |this, cx| this.render_message(ix, window, cx))
-                        .unwrap()
-                }
-            }),
+            list_state: list_state.clone(),
+            scrollbar_state: ScrollbarState::new(list_state),
             editing_message: None,
             last_error: None,
             pop_ups: Vec::new(),
@@ -1749,13 +1752,48 @@ impl ActiveThread {
                 .ok();
         }
     }
+
+    fn render_vertical_scrollbar(&self, cx: &mut Context<Self>) -> Stateful<Div> {
+        div()
+            .occlude()
+            .id("active-thread-scrollbar")
+            .on_mouse_move(cx.listener(|_, _, _, cx| {
+                cx.notify();
+                cx.stop_propagation()
+            }))
+            .on_hover(|_, _, cx| {
+                cx.stop_propagation();
+            })
+            .on_any_mouse_down(|_, _, cx| {
+                cx.stop_propagation();
+            })
+            .on_mouse_up(
+                MouseButton::Left,
+                cx.listener(|_, _, _, cx| {
+                    cx.stop_propagation();
+                }),
+            )
+            .on_scroll_wheel(cx.listener(|_, _, _, cx| {
+                cx.notify();
+            }))
+            .h_full()
+            .absolute()
+            .right_1()
+            .top_1()
+            .bottom_0()
+            .w(px(12.))
+            .cursor_default()
+            .children(Scrollbar::vertical(self.scrollbar_state.clone()))
+    }
 }
 
 impl Render for ActiveThread {
     fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
         v_flex()
             .size_full()
+            .relative()
             .child(list(self.list_state.clone()).flex_grow())
             .children(self.render_confirmations(cx))
+            .child(self.render_vertical_scrollbar(cx))
     }
 }