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>

1 files changed, 39 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}"
+            );
+        }
+    }
+}