Column shell: a drag that comes back where it set off gives the width the owner had, where the last width it reported stayed; the width a drag sets off from lives with its press, so each press starts afresh

AGENTS.md

@@ -221,7 +221,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - `typography::Ellipsis` shapes its line with the text style's own run, so a strike or an underline set on its box shows (`lists::ListItem::struck`, a closed task's title).
 - Focus that must follow an item to another parent is taken by the owner and keyed by the item: a handle gpui makes for an element dies with its path (`project::KanbanBoard`, whose cards keep focus across columns).
 - The theme's `platform`, the system's to start, decides the window buttons Ely draws, the title bar's layout and key caps, `secondary` included (`typography::keys::keystroke`). A title bar whose buttons the OS draws follows the OS.
-- A window whose sidebar runs to its top is `shell::ColumnShell`: each column's header drags the window; the first leaves the traffic lights room, and drawn controls lead the first header in the macOS manner or end the last. A set width is a flex basis, not a width, so it holds its header's own parts, which a width let run past it, and yields when the shell runs short; the host's header items keep one control's width and scroll when short. A resizable column drags by its border toward the filling column, its right before it and its left after it; the handle draws that border and occludes the header under it, and a drag sets off from the wider of the width drawn and the owner's, since a header's parts may hold a column wider and a short shell squeeze it narrower; a drag that has not moved sideways changes nothing.
+- A window whose sidebar runs to its top is `shell::ColumnShell`: each column's header drags the window; the first leaves the traffic lights room, and drawn controls lead the first header in the macOS manner or end the last. A set width is a flex basis, not a width, so it holds its header's own parts, which a width let run past it, and yields when the shell runs short; the host's header items keep one control's width and scroll when short. A resizable column drags by its border toward the filling column, its right before it and its left after it; the handle draws that border and occludes the header under it, and a drag sets off from the wider of the width drawn and the owner's, since a header's parts may hold a column wider and a short shell squeeze it narrower; a drag that has not moved sideways changes nothing, and one that comes back where it set off gives the width the owner had.
 - Taffy counts a flex item's text in its least width whatever width the item sets. A box that must add none lays its content absolutely inside a filling box (`shell::ColumnShell`'s title and body).
 - A centered title sits between a start and an end that grow alike from a zero basis, their padding on a box inside them, since taffy counts padding in the flex base; their content is their least width, so the title gives way before either end (`shell::TitleBar`).
 - A pane the window's blur shows through is `shell::Vibrancy`, filled with the palette's `glass`, in a window opened `WindowBackgroundAppearance::Blurred`: vibrancy on macOS, acrylic on Windows. gpui's `MicaBackdrop` and `MicaAltBackdrop` are DWM backdrops from Windows 11 build 22621; older builds get none, in silence, and macOS plain transparency.

src/shell/columns.rs

@@ -1,4 +1,4 @@
-use std::{cell::Cell, rc::Rc};
+use std::rc::Rc;
 
 use gpui::{
     AnyElement, App, Axis, DefiniteLength, Div, DragMoveEvent, ElementId, EmptyView, EntityId,
@@ -31,11 +31,17 @@ enum Side {
     Right,
 }
 
+#[derive(Clone, Copy)]
 struct BorderDrag {
     owner: EntityId,
     side: Side,
-    /// The width drawn when the drag set off.
-    start: Rc<Cell<Option<Pixels>>>,
+}
+
+/// Where a press on a border landed; once it moves sideways, the width it sets off from and the owner's then.
+#[derive(Clone, Copy)]
+struct Press {
+    at: Pixels,
+    start: Option<(Pixels, Pixels)>,
 }
 
 /// One column of a `ColumnShell`: a header that drags the window, then its body.
@@ -292,13 +298,9 @@ fn resizing(
         max,
         on_resize,
     } = resize;
-    let pressed = window.use_keyed_state((id.clone(), "press"), cx, |_, _| None::<Pixels>);
+    let pressed = window.use_keyed_state((id.clone(), "press"), cx, |_, _| None::<Press>);
     let owner = pressed.entity_id();
-    let drag = BorderDrag {
-        owner,
-        side,
-        start: Rc::default(),
-    };
+    let drag = BorderDrag { owner, side };
     let at = pressed.clone();
     let handle = resize_handle((id, "border"), Axis::Horizontal, cx)
         .absolute()
@@ -311,33 +313,48 @@ fn resizing(
         .occlude()
         .debug_selector(move || format!("column-shell-border-{ix}"))
         .on_mouse_down(MouseButton::Left, move |event: &MouseDownEvent, _, cx| {
-            at.update(cx, |at, _| *at = Some(event.position.x))
+            at.update(cx, |at, _| {
+                *at = Some(Press {
+                    at: event.position.x,
+                    start: None,
+                })
+            })
         })
         .on_drag(drag, |_, _, _, cx| cx.new(|_| EmptyView));
     box_.on_drag_move(move |event: &DragMoveEvent<BorderDrag>, window, cx| {
-        let drag = event.drag(cx);
-        if drag.owner != owner {
+        let BorderDrag {
+            owner: dragged,
+            side,
+        } = *event.drag(cx);
+        if dragged != owner {
             return;
         }
-        let Some(anchor) = *pressed.read(cx) else {
+        let Some(press) = *pressed.read(cx) else {
             log::error!("column shell: a drag on column {ix} with no press behind it");
             return;
         };
-        // A header's parts may hold a column wider, a short shell squeeze it narrower.
-        let start = drag
-            .start
-            .get()
-            .unwrap_or(event.bounds.size.width.max(width));
-        drag.start.set(Some(start));
-        let moved = event.event.position.x - anchor;
-        if moved == Pixels::ZERO {
-            return;
-        }
-        let next = match drag.side {
-            Side::Left => start - moved,
-            Side::Right => start + moved,
-        }
-        .clamp(min, max);
+        let moved = event.event.position.x - press.at;
+        let (start, kept) = match press.start {
+            Some(start) => start,
+            // A drag that has not moved sideways yet changes nothing.
+            None if moved == Pixels::ZERO => return,
+            // A header's parts may hold a column wider, a short shell squeeze it narrower.
+            None => {
+                let start = (event.bounds.size.width.max(width), width);
+                pressed.update(cx, |held, _| {
+                    *held = Some(Press {
+                        start: Some(start),
+                        ..press
+                    })
+                });
+                start
+            }
+        };
+        let next = match side {
+            _ if moved == Pixels::ZERO => kept,
+            Side::Left => (start - moved).clamp(min, max),
+            Side::Right => (start + moved).clamp(min, max),
+        };
         if next != width {
             on_resize(next, window, cx);
         }

src/shell/tests/border.rs

@@ -62,20 +62,27 @@ fn column(ix: usize, cx: &mut VisualTestContext) -> gpui::Bounds<Pixels> {
     cx.debug_bounds(name).expect("a column")
 }
 
-/// Presses at `from`, moves past gpui's drag threshold, then by `by`, and lets go.
-fn drag(from: Point<Pixels>, by: Pixels, cx: &mut VisualTestContext) {
+/// Presses at `from`, moves past gpui's drag threshold, then to each offset `path` names, and lets go.
+fn drag_path(from: Point<Pixels>, path: &[Pixels], cx: &mut VisualTestContext) {
     let none = Modifiers::none();
     cx.simulate_mouse_down(from, MouseButton::Left, none);
     let set_off = from + point(px(0.0), px(4.0));
     cx.simulate_mouse_move(set_off, Some(MouseButton::Left), none);
     cx.run_until_parked();
-    let to = set_off + point(by, px(0.0));
-    cx.simulate_mouse_move(to, Some(MouseButton::Left), none);
-    cx.run_until_parked();
+    let mut to = set_off;
+    for by in path {
+        to = set_off + point(*by, px(0.0));
+        cx.simulate_mouse_move(to, Some(MouseButton::Left), none);
+        cx.run_until_parked();
+    }
     cx.simulate_mouse_up(to, MouseButton::Left, none);
     cx.run_until_parked();
 }
 
+fn drag(from: Point<Pixels>, by: Pixels, cx: &mut VisualTestContext) {
+    drag_path(from, &[by], cx);
+}
+
 fn heard(view: &Entity<Sided>, cx: &mut VisualTestContext) -> Option<Pixels> {
     view.read_with(cx, |view, _| view.heard.last().copied())
 }
@@ -202,3 +209,56 @@ fn a_squeezed_column_drags_from_the_owner_s_width(cx: &mut TestAppContext) {
         "the right border moves left"
     );
 }
+
+/// A drag that comes back where it set off gives the width the owner had, a held or squeezed one too.
+#[gpui::test]
+fn a_drag_back_where_it_set_off_gives_the_width_it_had(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let cases = [
+        (
+            Platform::Linux,
+            px(1000.0),
+            [px(200.0), px(200.0)],
+            0,
+            px(50.0),
+        ),
+        (
+            Platform::Linux,
+            px(1000.0),
+            [px(200.0), px(200.0)],
+            2,
+            px(-50.0),
+        ),
+        (
+            Platform::Windows,
+            px(1000.0),
+            [px(200.0), px(100.0)],
+            2,
+            px(-10.0),
+        ),
+        (
+            Platform::Linux,
+            px(700.0),
+            [px(350.0), px(350.0)],
+            0,
+            px(20.0),
+        ),
+    ];
+    for (platform, width, widths, ix, by) in cases {
+        let (view, cx) = cx.add_window_view(move |_, _| Sided {
+            platform,
+            width,
+            widths,
+            heard: Vec::new(),
+        });
+        cx.run_until_parked();
+        let name = ["column-shell-border-0", "", "column-shell-border-2"][ix];
+        let border = cx.debug_bounds(name).expect("a border").center();
+        let drawn = column(ix, cx);
+        drag_path(border, &[by, px(0.0)], cx);
+        let heard = view.read_with(cx, |view, _| view.heard.clone());
+        assert_eq!(heard.len(), 2, "{platform:?} {ix}: out and back, {heard:?}");
+        assert_eq!(heard[1], widths[ix / 2], "{platform:?} {ix}");
+        assert_eq!(column(ix, cx), drawn, "{platform:?} {ix}: drawn as before");
+    }
+}