Pin Tab's rightmost-column clamp and bound the tile-bar walks

improvise-qnry: a lockstep case compares Tab at the rightmost column with
the host's real CommitAndAdvance, and the tile lockstep case now presses
past the last tile. improvise-ybum: browser_round_trip's tile and column
walks are bounded and fail on a stuck press instead of hanging.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/test-evidence/clients.md

@@ -3819,6 +3819,23 @@ ticket's transcript spells the route as `T r l c Esc`, which depends on
 `state().tiles` and walks to it, so the test pins *the route exists*, not a
 particular ordering the producer is free to change.
 
+**The walks are bounded (improvise-ybum),** found in the adversarial review
+of PR #256. `tile_to` and the column walk were `while` loops that pressed `l`
+or `h` until an index was reached. A resolver that stopped the cursor short
+never ended the loop: under the left-clamp fault a reviewer killed the binary
+after 17 minutes, and in CI a hang costs the job timeout and names no test.
+Both loops now go through `walk_to`. It presses at most `limit` times (the tile
+count plus one, or the column count plus one), and it fails as soon as a press
+leaves the index where it was, naming the stuck index and the target.
+
+- **Rival `left_clamp_one`** (`clamp(1, ..)` in `tile_cursor`): before the
+  change, the binary was still running at a 90-second `timeout` and was killed
+  (exit 124). After it, the test fails in 0.04s with "the tile cursor is stuck
+  at 1: `h` did not move it toward 0".
+- **Rival `column_cap_one`** (`clamp(0, col_max.min(1))` in the grid move):
+  fails in 0.04s with "the column cursor is stuck at 1: `l` did not move it
+  toward 2", so the column walk is bounded too, not only the tile walk.
+
 ### opening_an_editor_is_declined_off_the_grid_and_not_off_the_mode
 
 improvise-bd9o, and it exists because **the route test could not state the
@@ -5150,3 +5167,42 @@ when the index does not move (gotcha 47).
   `SetTileCatIdx(3)` over three tiles.
 - The control matters: without the unclamped move first, a resolver that never
   moves at all would also pass the two clamped presses.
+
+### tab_at_the_rightmost_column_stays_there_on_both_ends
+
+improvise-qnry, found in the adversarial review of PR #256. Dropping the clamp
+from `advanced`'s `Advance::Right` arm, so that it reads `Some((row, col + 1))`,
+passed the whole workspace (1942 tests). `tab_in_an_editor_commits_and_lands_one_column_right`
+sits on a centred cursor on purpose, so it cannot see the edge. Without the
+clamp, Tab at the rightmost column of a pivot grid sends the selection one
+column past the grid, and the editor opens on no cell.
+
+This is a lockstep case: the client is compared with the host's real
+`CommitAndAdvance`, not with a hand-written expectation. The fixture is
+`precise_host`, two rows by two columns, with the cursor at (1, 1). That is the
+rightmost column at a nonzero row, so "the last column" is not "the first
+cell". The test presses `i` then `Tab` and compares the cursor and the mode on
+both ends after each key. Its preconditions assert that the grid is not in
+records mode, where this corner declines instead, and that the editor is open
+again at the end.
+
+- **Rival `right_unclamped`** (the ticket's fault): fails only this test across
+  the workspace (1942 passed, 1 failed). The client lands on `(1, 2)` with
+  `col_offset: 1`; the host stays on `(1, 1)`.
+- improvise-3tt4's acceptance test (Tab at the rightmost column, comparing
+  buffers) would probably also see this fault, but it lives on PR #227's branch,
+  which is not in this base. This test compares the selection and does not
+  depend on #227.
+
+### a_locally_resolved_tile_cursor_agrees_with_the_host
+
+Extended for improvise-qnry. The test used to press `l` three times over
+`tall_host`'s four tiles, starting at 0, so it never reached the upper clamp:
+any rival that differed only at the last tile passed it. It now presses
+`count + 1` times and asserts it ends on the last tile, so the final press is
+checked against the host's real `MoveTileCursor` clamp.
+
+- **Rival `wraparound`** (`.rem_euclid(count)` in place of the clamp): fails
+  this test at press 3 (client 0, host 3), and also the unit test
+  `resolve::tests::the_tile_cursor_stops_on_the_last_tile`. Nothing else in
+  the workspace fails (1941 passed, 2 failed).

crates/improvise-wasm-client/tests/browser_round_trip.rs

@@ -431,15 +431,44 @@ fn tile_to(
         .iter()
         .position(|t| t.category == category)
         .unwrap_or_else(|| panic!("the tile bar must carry {category}"));
-    while client.state().view.tile_cat_idx < at {
-        press(host, id, client, "l");
-    }
-    while client.state().view.tile_cat_idx > at {
-        press(host, id, client, "h");
-    }
+    let limit = client.state().tiles.tiles.len() + 1;
+    walk_to(host, id, client, "the tile cursor", at, limit, |c| {
+        c.state().view.tile_cat_idx
+    });
     press(host, id, client, key);
 }
 
+/// Press `l` or `h` until `read` gives `target`, failing on a stuck press or
+/// after `limit` presses rather than hanging (improvise-ybum). Evidence file.
+fn walk_to(
+    host: &mut Host,
+    id: improvise_command::ui::project::SessionId,
+    client: &mut Client,
+    what: &str,
+    target: usize,
+    limit: usize,
+    read: impl Fn(&Client) -> usize,
+) {
+    for _ in 0..limit {
+        let now = read(client);
+        if now == target {
+            return;
+        }
+        let key = if now < target { "l" } else { "h" };
+        press(host, id, client, key);
+        assert_ne!(
+            read(client),
+            now,
+            "{what} is stuck at {now}: `{key}` did not move it toward {target}"
+        );
+    }
+    assert_eq!(
+        read(client),
+        target,
+        "{what} did not reach {target} in {limit} presses"
+    );
+}
+
 /// `toggle-records-mode` is declined and the tile bar reaches records mode
 /// anyway (improvise-bd9o). Evidence file.
 #[test]
@@ -472,9 +501,15 @@ fn the_tile_bar_route_into_records_mode_does_not_strand_the_user() {
         "the fixture must reach past the coordinate column"
     );
     for col in 0..cols {
-        while client.state().view.cursor().selected.1 < col {
-            press(&mut host, id, &mut client, "l");
-        }
+        walk_to(
+            &mut host,
+            id,
+            &mut client,
+            "the column cursor",
+            col,
+            cols + 1,
+            |c| c.state().view.cursor().selected.1,
+        );
         let kind = format!(
             "{:?}",
             client.state().grid.cell(0, col).and_then(|c| c.id.clone())

crates/improvise-wasm-client/tests/lockstep.rs

@@ -462,7 +462,8 @@ fn a_locally_resolved_tile_cursor_agrees_with_the_host() {
         "a one-tile fixture clamps immediately and proves nothing"
     );
 
-    for press in 0..3 {
+    let count = client.tiles.tiles.len();
+    for press in 0..=count {
         let event = KeyEvent::new(KeyCode::Char('l'), KeyModifiers::NONE);
         let KeyOutcome::Local(cmds) = resolve(&client, event) else {
             panic!("press {press}: the tile cursor must resolve locally");
@@ -476,7 +477,11 @@ fn a_locally_resolved_tile_cursor_agrees_with_the_host() {
             "press {press}: client and host disagree about the tile cursor"
         );
     }
-    assert!(client.view.tile_cat_idx > 0, "the fixture moved nothing");
+    assert_eq!(
+        client.view.tile_cat_idx,
+        count - 1,
+        "count + 1 presses must end clamped on the last tile (improvise-qnry)"
+    );
 }
 
 // The editing path (improvise-bpq2's second half).
@@ -671,6 +676,50 @@ fn a_locally_committed_edit_leaves_both_ends_in_the_same_place() {
     );
 }
 
+/// improvise-qnry: Tab at the rightmost column clamps there on both ends, as
+/// `CommitAndAdvance` does. Evidence: `context/test-evidence/clients.md`.
+#[test]
+fn tab_at_the_rightmost_column_stays_there_on_both_ends() {
+    let window = (2, 2);
+    let mut app = precise_host(improvise_protocol::CellValue::Number(4.0));
+    let mut client = client_ready_to_edit(&app, window);
+    let extent = client.grid.extent;
+    assert!(
+        !client.grid.records,
+        "records mode declines this corner instead"
+    );
+    assert_eq!(
+        client.view.cursor().selected,
+        (1, extent.cols - 1),
+        "the cursor must start on the rightmost column at a nonzero row"
+    );
+
+    for key in ["i", "Tab"] {
+        let event = BrowserKey::new(key).to_key_event().unwrap();
+        let KeyOutcome::Local(cmds) = resolve(&client, event) else {
+            panic!("{key}: the client must answer this key");
+        };
+        for cmd in &cmds {
+            client.fold(cmd);
+        }
+        reduce_key(&mut app, event);
+        assert_eq!(
+            client.view.cursor(),
+            app.view_state.cursor(),
+            "{key}: client and host disagree about the cursor"
+        );
+        assert_eq!(
+            format!("{:?}", client.view.mode),
+            format!("{:?}", app.view_state.mode),
+            "{key}: client and host disagree about the mode"
+        );
+    }
+    assert!(
+        app.view_state.mode.is_editing(),
+        "Tab must have reopened the editor, or the walk stopped short"
+    );
+}
+
 // The second tranche (improvise-e24v).
 
 /// A document with two categories, two views and a value, so every widget the