Files: a narrow listing keeps its names; a column starts fresh for a new folder

The directory listing's size and date columns are fixed, so at 280px the
name had no room and rows showed no names. The heading and rows now sit
in a sideways scroll box whose child keeps the least width: the name's
least, the fixed columns and each row's icon, gaps and padding. A column
of the explorer was keyed by its depth, so a new folder there inherited
the old list's cursor; columns and rows are keyed by their folder. The
explorer asserts each folder on its path is a folder in its parent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/files/tests/explorer.rs

@@ -174,3 +174,83 @@ fn the_pick_is_in_the_folder_shown() {
     let here = vec![DirEntry::file("Brief.md", 10, then())];
     let _ = FileExplorer::new("explorer", ["Home"], [here], FileView::List).selected("Ghost.md");
 }
+
+/// An explorer that goes where it is sent, over a small tree, and the files it opened.
+struct Walking {
+    path: Vec<SharedString>,
+    opened: Vec<SharedString>,
+}
+
+fn folder(path: &[SharedString]) -> Vec<DirEntry> {
+    let names: Vec<&str> = path.iter().map(|level| level.as_ref()).collect();
+    match names.join("/").as_str() {
+        "Home" => vec![
+            DirEntry::folder("Projects", then()),
+            DirEntry::folder("Documents", then()),
+        ],
+        "Home/Projects" => vec![
+            DirEntry::folder("Atrium", then()),
+            DirEntry::folder("Dunes", then()),
+        ],
+        "Home/Projects/Dunes" => vec![DirEntry::file("Survey.csv", 10, then())],
+        "Home/Documents" => vec![
+            DirEntry::file("Contract.docx", 10, then()),
+            DirEntry::file("Invoice.pdf", 10, then()),
+        ],
+        other => panic!("no folder {other}"),
+    }
+}
+
+impl Render for Walking {
+    fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        let (go, open) = (cx.entity(), cx.entity());
+        let levels = (1..=self.path.len()).map(|depth| folder(&self.path[..depth]));
+        let explorer = FileExplorer::new("explorer", self.path.clone(), levels, FileView::Columns)
+            .on_navigate(move |path, _, cx| {
+                go.update(cx, |host, cx| {
+                    host.path = path.to_vec();
+                    cx.notify();
+                })
+            })
+            .on_open(move |entry, _, cx| {
+                open.update(cx, |host, _| host.opened.push(entry.name().clone()))
+            });
+        div().w(px(640.0)).child(explorer)
+    }
+}
+
+#[gpui::test]
+fn a_new_folder_at_a_depth_starts_its_column_fresh(cx: &mut TestAppContext) {
+    setup(cx);
+    let (host, cx) = cx.add_window_view(|_, _| Walking {
+        path: ["Home", "Projects"].map(SharedString::from).to_vec(),
+        opened: Vec::new(),
+    });
+    cx.update(|window, _| window.activate_window());
+    settle(cx);
+    tab_to(6, cx);
+    press("down", cx);
+    host.update(cx, |host, cx| {
+        host.path = ["Home", "Documents"].map(SharedString::from).to_vec();
+        cx.notify();
+    });
+    settle(cx);
+    tab_to(6, cx);
+    press("enter", cx);
+    assert_eq!(
+        host.read_with(cx, |host, _| host.opened.clone()),
+        ["Contract.docx"]
+    );
+}
+
+#[test]
+#[should_panic(expected = "Projects is no folder in Home")]
+fn each_folder_on_the_path_is_listed_in_its_parent() {
+    let root = vec![DirEntry::file("Projects", 10, then())];
+    let _ = FileExplorer::new(
+        "explorer",
+        ["Home", "Projects"],
+        [root, Vec::new()],
+        FileView::List,
+    );
+}