T58: the terminal shows a scrollbar over its history, which a user asked for in issue #2 after 0.1.5, since a program that takes the mouse takes the wheel: a thumb along the right edge, in Ely's sizes and colors, stands where the view stands, and a drag moves the view through the history; none shows without history

src/term/mod.rs

@@ -16,6 +16,7 @@ mod pointer;
 mod prompts;
 mod pty;
 mod reports;
+mod scrollbar;
 pub mod shell;
 mod status;
 mod tap;

src/term/scrollbar.rs

@@ -0,0 +1,170 @@
+//! The terminal's scrollbar: a thumb over the history, in Ely's sizes and colors, that a drag moves through it, whatever the wheel is bound to.
+
+use std::{cell::Cell, rc::Rc};
+
+use alacritty_terminal::{
+    grid::{Dimensions, Scroll},
+    term::Term,
+};
+use ely_gpui_component::theme::ActiveTheme;
+use gpui::{
+    AnyElement, AppContext as _, Context, DragMoveEvent, EmptyView, EntityId, InteractiveElement,
+    IntoElement, ParentElement, Pixels, StatefulInteractiveElement, Styled, Window, div, relative,
+};
+
+use super::view::TerminalView;
+
+/// A drag of the thumb: its view, how far down the history the view stood as it began, and the drag's first point.
+pub(super) struct ThumbDrag {
+    owner: EntityId,
+    from: f32,
+    anchor: Rc<Cell<Option<Pixels>>>,
+}
+
+/// Where the view stands in the history: the lines above the screen, the screen's lines, and how far up the view is.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub(super) struct Span {
+    history: usize,
+    screen: usize,
+    offset: usize,
+}
+
+impl Span {
+    pub(super) fn of<T>(term: &Term<T>) -> Self {
+        Self {
+            history: term.grid().history_size(),
+            screen: term.screen_lines(),
+            offset: term.grid().display_offset(),
+        }
+    }
+
+    /// The thumb's share of the track.
+    fn share(self) -> f32 {
+        self.screen as f32 / (self.history + self.screen) as f32
+    }
+
+    /// How far down the history the view stands: 0 at its top, 1 at the bottom.
+    fn ratio(self) -> f32 {
+        1.0 - self.offset as f32 / self.history as f32
+    }
+
+    /// How far up the view stands with the thumb `ratio` down the track.
+    fn offset_at(self, ratio: f32) -> usize {
+        ((1.0 - ratio.clamp(0.0, 1.0)) * self.history as f32).round() as usize
+    }
+}
+
+/// The bar along the view's right edge, over the padding there; none without history, as on the alternate screen.
+pub(super) fn bar(span: Span, window: &Window, cx: &Context<TerminalView>) -> Option<AnyElement> {
+    if span.history == 0 {
+        return None;
+    }
+    let theme = cx.theme();
+    let rem = window.rem_size();
+    let thickness = theme.scrollbar_thickness().to_pixels(rem);
+    let min_thumb = theme.scrollbar_min_thumb().to_pixels(rem);
+    let (idle, busy) = (theme.colors.fg.opacity(0.22), theme.colors.fg.opacity(0.42));
+    let ratio = span.ratio();
+    // The room above and below the thumb shares what it leaves as the view stands.
+    let (mut above, mut below) = (div(), div());
+    above.style().flex_grow = Some(ratio);
+    below.style().flex_grow = Some(1.0 - ratio);
+    let drag = ThumbDrag {
+        owner: cx.entity_id(),
+        from: ratio,
+        anchor: Rc::default(),
+    };
+    let thumb = div()
+        .id("thumb")
+        .w(thickness)
+        .h(relative(span.share()))
+        .min_h(min_thumb)
+        .flex_shrink_0()
+        .rounded_full()
+        .bg(idle)
+        .hover(|style| style.bg(busy))
+        .on_drag(drag, |_, _, _, cx| cx.new(|_| EmptyView));
+    let bar = div()
+        .id("scrollbar")
+        .absolute()
+        .top_0()
+        .bottom_0()
+        .right_0()
+        .w(thickness * 2.0)
+        .pl(thickness * 0.5)
+        .flex()
+        .flex_col()
+        .occlude()
+        .on_drag_move(cx.listener(TerminalView::drag_thumb))
+        .child(above)
+        .child(thumb)
+        .child(below);
+    Some(bar.into_any_element())
+}
+
+impl TerminalView {
+    /// Scrolls the history after the thumb, along its track's bounds.
+    fn drag_thumb(
+        &mut self,
+        event: &DragMoveEvent<ThumbDrag>,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        let drag = event.drag(cx);
+        if drag.owner != cx.entity_id() {
+            return;
+        }
+        let min_thumb = cx
+            .theme()
+            .scrollbar_min_thumb()
+            .to_pixels(window.rem_size());
+        let mut term = self.pty.term.lock();
+        let span = Span::of(&term);
+        let height = event.bounds.size.height;
+        let travel = height - (height * span.share()).max(min_thumb).min(height);
+        if span.history == 0 || travel <= Pixels::ZERO {
+            return;
+        }
+        let along = event.event.position.y - event.bounds.origin.y;
+        let anchor = drag.anchor.get().unwrap_or(along);
+        drag.anchor.set(Some(anchor));
+        let offset = span.offset_at(drag.from + (along - anchor) / travel);
+        let delta = offset as i32 - span.offset as i32;
+        if delta != 0 {
+            term.scroll_display(Scroll::Delta(delta));
+            drop(term);
+            log::trace!(
+                "terminal {}: the scrollbar shows {offset} lines up",
+                self.pty.pid
+            );
+            cx.notify();
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn the_thumb_stands_where_the_view_stands_in_the_history() {
+        let span = |offset| Span {
+            history: 300,
+            screen: 100,
+            offset,
+        };
+        assert_eq!(span(0).share(), 0.25, "a screen of four");
+        assert_eq!(span(0).ratio(), 1.0, "at the bottom");
+        assert_eq!(span(300).ratio(), 0.0, "at the top");
+        assert_eq!(span(150).ratio(), 0.5);
+        assert_eq!(
+            span(0).offset_at(0.0),
+            300,
+            "the thumb at the top shows the oldest lines"
+        );
+        assert_eq!(span(0).offset_at(1.0), 0);
+        assert_eq!(span(0).offset_at(0.5), 150);
+        assert_eq!(span(0).offset_at(-0.2), 300, "past the track's ends");
+        assert_eq!(span(0).offset_at(1.3), 0);
+    }
+}

src/term/view.rs

@@ -28,6 +28,7 @@ use super::{
     pointer::Pointer,
     pty::{Events, Pty, PtyEvent, config, fit},
     reports::Notices,
+    scrollbar::{self, Span},
     status::{Records, State},
     zoom,
 };
@@ -413,6 +414,7 @@ impl Render for TerminalView {
         });
         let shown = frame(&term, &ink, &lit);
         let blinks = term.cursor_style().blinking;
+        let span = Span::of(&term);
         drop(term);
         let focused = self.focus.is_focused(window) && window.is_window_active();
         self.blinking = focused && blinks;
@@ -480,9 +482,14 @@ impl Render for TerminalView {
                 root.text_color(ink.fg).child(format!("{error:#}"))
             }
         };
-        div().relative().size_full().child(root).children(
-            self.find_box(cx)
-                .map(|found| div().absolute().top_2().right_4().child(found)),
-        )
+        div()
+            .relative()
+            .size_full()
+            .child(root)
+            .children(scrollbar::bar(span, window, cx))
+            .children(
+                self.find_box(cx)
+                    .map(|found| div().absolute().top_2().right_4().child(found)),
+            )
     }
 }