fix: reveal the full last tab in overflowing tab strips (#4152)

refs #4151

Co-authored-by: akbash-bot <300245827+akbash-bot@users.noreply.github.com>

2 files changed, 137 insertions, 17 deletions

diff --git a/src/client/shell/tabs.rs b/src/client/shell/tabs.rs
index 9e9901ff..3fdcf7dc 100644
--- a/src/client/shell/tabs.rs
+++ b/src/client/shell/tabs.rs
@@ -134,7 +134,7 @@ pub(crate) fn render_tab_bar(
 
     if overflow && mouse_chrome {
         hits.tab_scroll_right = Rect::new(tab_right, area.y, TAB_SCROLL_BUTTON_WIDTH, 1);
-        let can_scroll_right = last_visible.is_some_and(|index| index + 1 < tabs.len());
+        let can_scroll_right = *tab_scroll < max_scroll;
         put_text(
             buffer,
             hits.tab_scroll_right.x,
@@ -353,25 +353,22 @@ fn centered_tab_scroll(focused: usize, widths: &[u16], available: u16) -> usize
 }
 
 fn max_tab_scroll(widths: &[u16], available: u16) -> usize {
-    (0..widths.len())
-        .find(|start| last_visible_tab(*start, widths, available) == widths.len().checked_sub(1))
-        .unwrap_or(0)
-}
-
-fn last_visible_tab(start: usize, widths: &[u16], available: u16) -> Option<usize> {
-    let mut remaining = available;
-    let mut last = None;
-    for (index, width) in widths.iter().copied().enumerate().skip(start) {
-        if remaining == 0 {
-            break;
-        }
-        last = Some(index);
-        if width >= remaining {
+    let Some((&last, preceding)) = widths.split_last() else {
+        return 0;
+    };
+    let mut start = preceding.len();
+    let mut used = u32::from(last);
+    // Keep the longest fully visible suffix, not merely a sliver of the last tab.
+    // An oversized last tab must still be reachable at the start of the strip.
+    for width in preceding.iter().rev() {
+        let required = used + 1 + u32::from(*width);
+        if required > u32::from(available) {
             break;
         }
-        remaining = remaining.saturating_sub(width.saturating_add(1));
+        used = required;
+        start -= 1;
     }
-    last
+    start
 }
 
 fn tab_label(tab: &ClientShellTab) -> String {
@@ -381,3 +378,28 @@ fn tab_label(tab: &ClientShellTab) -> String {
         tab.label.clone()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::max_tab_scroll;
+
+    #[test]
+    fn trailing_scroll_limit_accounts_for_full_widths_and_separators() {
+        for (widths, available, expected) in [
+            (&[][..], 0, 0),
+            (&[8, 13][..], 0, 1),
+            (&[8, 13][..], 1, 1),
+            (&[8, 13][..], 12, 1),
+            (&[8, 13][..], 21, 1),
+            (&[8, 13][..], 22, 0),
+            (&[8, 13][..], 30, 0),
+            (&[8, u16::MAX][..], u16::MAX, 1),
+        ] {
+            assert_eq!(
+                max_tab_scroll(widths, available),
+                expected,
+                "widths={widths:?}, available={available}"
+            );
+        }
+    }
+}
diff --git a/src/client/shell/tests/chrome_context.rs b/src/client/shell/tests/chrome_context.rs
index 5747bb97..ed21bc8a 100644
--- a/src/client/shell/tests/chrome_context.rs
+++ b/src/client/shell/tests/chrome_context.rs
@@ -47,6 +47,104 @@ fn tab_overflow_controls_scroll_the_client_owned_tab_bar() {
 }
 
 #[test]
+fn focused_last_overflow_tab_shows_its_full_label() {
+    let mut projected = snapshot();
+    let labels = [
+        "1",
+        "Laiza Portfolio Site",
+        "linkedin posts",
+        "update cv",
+        "nvim",
+        "brother",
+        "day organiser",
+        "nvim test",
+    ];
+    projected.tabs = labels
+        .iter()
+        .enumerate()
+        .map(|(index, label)| ClientShellTab {
+            tab_id: format!("tab_{}", index + 1),
+            workspace_id: "ws_1".into(),
+            number: index + 1,
+            label: (*label).into(),
+            custom_label: index > 0,
+            zoomed: false,
+            focused: index == 7,
+            agent_status: AgentStatus::Idle,
+        })
+        .collect();
+    let mut state = ClientShellState::new(ClientShellConfig::from_config(&Config::default()));
+    for number in [8, 7, 8] {
+        let tab_id = format!("tab_{number}");
+        projected.focused_tab_id = Some(tab_id.clone());
+        projected.workspaces[0].active_tab_id = tab_id.clone();
+        projected.panes[0].tab_id = tab_id.clone();
+        for tab in &mut projected.tabs {
+            tab.focused = tab.tab_id == tab_id;
+        }
+        state.set_snapshot(Box::new(projected.clone()));
+        state.set_pane_surface(surface());
+        let frame = state
+            .compose(133, 20)
+            .expect("reporter's overflowing strip");
+        assert_eq!(
+            state.hits.new_tab.right() - state.hits.tab_scroll_left.x,
+            107
+        );
+        let rect = state
+            .hits
+            .tabs
+            .iter()
+            .find(|(_, id)| id == &tab_id)
+            .expect("focused tab")
+            .0;
+        let text = (rect.x..rect.right())
+            .map(|x| {
+                frame.cells[(rect.y * frame.width + x) as usize]
+                    .symbol
+                    .as_str()
+            })
+            .collect::<String>();
+        assert!(
+            text.contains(labels[number - 1]),
+            "focused tab rendered as {text:?}, rect={rect:?}"
+        );
+    }
+
+    // Manual scrolling must be able to reveal the rest of a partially drawn last tab.
+    let end_scroll = state.tab_scroll;
+    let left = state.hits.tab_scroll_left;
+    state.handle_raw_events(vec![RawInputEvent::Mouse(MouseEvent {
+        kind: MouseEventKind::Down(MouseButton::Left),
+        column: left.x + 1,
+        row: left.y,
+        modifiers: KeyModifiers::empty(),
+    })]);
+    let frame = state.compose(133, 20).expect("manual scroll left");
+    assert_eq!(state.tab_scroll, end_scroll - 1);
+    let right = state.hits.tab_scroll_right;
+    assert_eq!(
+        frame.cells[(right.y * frame.width + right.x + 1) as usize].fg,
+        crate::protocol::color_to_u32(state.config.palette.overlay1),
+        "right arrow stays enabled while the final tab is clipped"
+    );
+    state.handle_raw_events(vec![RawInputEvent::Mouse(MouseEvent {
+        kind: MouseEventKind::Down(MouseButton::Left),
+        column: right.x + 1,
+        row: right.y,
+        modifiers: KeyModifiers::empty(),
+    })]);
+    let frame = state.compose(133, 20).expect("manual scroll right");
+    assert_eq!(state.tab_scroll, end_scroll);
+    assert!(frame_rows(&frame)[0].contains("nvim test"));
+    assert_eq!(
+        frame.cells[(right.y * frame.width + right.x + 1) as usize].fg,
+        crate::protocol::color_to_u32(state.config.palette.overlay0),
+        "right arrow dims at the useful scroll limit"
+    );
+}
+
+#[test]
 fn focused_workspace_change_reveals_new_workspace_in_full_sidebar() {
     let mut initial = snapshot();
     let template = initial.workspaces[0].clone();