review(p9-fb-10-final): extract place_cursor_x helper + filter overlay render test

- Add `place_cursor_x(inner_x, inner_width, prompt_w, cursor_col) -> u16`
  to `input.rs`: sums in `usize` (no u16 wrap), clamps to inner right
  edge, tries_into with u16::MAX fallback. Two unit tests pin the clamp
  and the in-bounds path.
- Re-export from `lib.rs` alongside `InputBuffer`, `display_width`,
  `truncate_to_display_width`.
- Replace the open-coded 2-line `raw_x`/`cursor_x` blocks in Search,
  Ask, and Library with a single `place_cursor_x` call each —
  consistent usize arithmetic across all three panes.
- Add `filter_overlay_render_places_cursor_on_focused_field` integration
  test in `tests/library.rs`: opens the filter overlay, renders through
  `TestBackend`, asserts `terminal.get_cursor_position().x > 0` (label
  offset > 0 proves `set_cursor_position` was called with a meaningful
  coordinate, not stuck at origin).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/kebab-tui/src/ask.rs

@@ -83,9 +83,10 @@ fn render_input(f: &mut Frame, area: Rect, s: &AskState, theme: &crate::theme::T
     // omits set_cursor_position (Library/Inspect), ratatui calls
     // hide_cursor instead. So this single call both positions and
     // unhides the caret for the Ask input column.
-    let prompt_w = crate::input::display_width(PROMPT) as u16;
-    let raw_x = inner.x + prompt_w + s.input.cursor_col() as u16;
-    let cursor_x = raw_x.min(inner.x + inner.width.saturating_sub(1));
+    // place_cursor_x sums in usize (avoiding u16 wrap) and clamps to
+    // the right edge of the inner area.
+    let prompt_w = crate::input::display_width(PROMPT);
+    let cursor_x = crate::input::place_cursor_x(inner.x, inner.width, prompt_w, s.input.cursor_col());
     f.set_cursor_position((cursor_x, inner.y));
 }
 

crates/kebab-tui/src/input.rs

@@ -32,6 +32,27 @@
 
 use unicode_width::{UnicodeWidthChar, UnicodeWidthStr};
 
+/// Compute the cursor column for a text-input pane: prompt width +
+/// content cursor, summed in `usize` to avoid `u16` overflow, then
+/// clamped to fit within `inner_width` columns from `inner_x`.
+///
+/// Use as:
+/// ```ignore
+/// f.set_cursor_position((place_cursor_x(inner.x, inner.width, prompt_w, buf.cursor_col()), inner.y));
+/// ```
+///
+/// If a fourth input pane is added, use this helper rather than
+/// open-coding the arithmetic — one place to fix if the clamping
+/// policy ever changes.
+pub fn place_cursor_x(inner_x: u16, inner_width: u16, prompt_w: usize, cursor_col: usize) -> u16 {
+    let raw = (inner_x as usize)
+        .saturating_add(prompt_w)
+        .saturating_add(cursor_col);
+    let max = (inner_x as usize)
+        .saturating_add(inner_width.saturating_sub(1) as usize);
+    raw.min(max).try_into().unwrap_or(u16::MAX)
+}
+
 /// Display width of `s` in terminal columns. CJK / fullwidth = 2
 /// per char, ASCII = 1, combining marks = 0. Sums every char's
 /// `unicode-width` reading — same calculation Ratatui uses
@@ -317,4 +338,18 @@ mod tests {
         assert!(b.is_empty());
         assert_eq!(b.cursor_col(), 0);
     }
+
+    /// p9-fb-10: place_cursor_x clamps within the inner area.
+    #[test]
+    fn place_cursor_x_clamps_to_inner_right_edge() {
+        // inner.x=10, width=20, so the rightmost column is 10+20-1 = 29.
+        // prompt_w=2, cursor_col=100 (overflow) → clamped to 29.
+        assert_eq!(place_cursor_x(10, 20, 2, 100), 29);
+    }
+
+    /// p9-fb-10: place_cursor_x preserves position when within bounds.
+    #[test]
+    fn place_cursor_x_keeps_position_when_within_bounds() {
+        assert_eq!(place_cursor_x(10, 20, 2, 5), 17); // 10 + 2 + 5
+    }
 }

crates/kebab-tui/src/lib.rs

@@ -27,7 +27,7 @@ mod search;
 mod terminal;
 mod theme;
 
-pub use input::{InputBuffer, display_width, truncate_to_display_width};
+pub use input::{InputBuffer, display_width, place_cursor_x, truncate_to_display_width};
 pub use theme::{Palette, Role, Theme};
 pub use app::{
     App, AskState, IngestState, InspectState, InspectTarget, KeyOutcome, LibraryState, Mode,

crates/kebab-tui/src/library.rs

@@ -153,13 +153,14 @@ fn render_filter_overlay(f: &mut Frame, area: Rect, edit: &FilterEdit, theme: &c
     // omits set_cursor_position (Library/Inspect main view), ratatui
     // calls hide_cursor instead. So this single call positions the
     // caret on the focused field of the filter overlay.
+    // place_cursor_x sums in usize (avoiding u16 wrap) and clamps to
+    // the right edge of the inner area.
     let (label, focused_buf, row_offset) = match edit.field {
         FilterField::Tags => (LABEL_TAGS, &edit.tags_buf, 0u16),
         FilterField::Lang => (LABEL_LANG, &edit.lang_buf, 1u16),
     };
-    let label_w = display_width(label) as u16;
-    let raw_x = inner.x + label_w + focused_buf.cursor_col() as u16;
-    let cursor_x = raw_x.min(inner.x + inner.width.saturating_sub(1));
+    let label_w = display_width(label);
+    let cursor_x = crate::input::place_cursor_x(inner.x, inner.width, label_w, focused_buf.cursor_col());
     f.set_cursor_position((cursor_x, inner.y + row_offset));
 }
 

crates/kebab-tui/src/search.rs

@@ -91,13 +91,10 @@ fn render_input_bar(f: &mut Frame, area: Rect, s: &SearchState, theme: &crate::t
     // omits set_cursor_position (Library/Inspect), ratatui calls
     // hide_cursor instead. So this single call both positions and
     // unhides the caret for the Search input column.
-    let raw_x = inner.x + (prompt_w + s.input.cursor_col()) as u16;
-    // Clamp to the right edge of the inner area — a long CJK query
-    // in a narrow terminal could otherwise place the caret beyond
-    // the box; crossterm passes coords through verbatim.
-    let cursor_x = raw_x.min(inner.x + inner.width.saturating_sub(1));
-    let cursor_y = inner.y;
-    f.set_cursor_position((cursor_x, cursor_y));
+    // place_cursor_x sums in usize (avoiding u16 wrap) and clamps to
+    // the right edge of the inner area.
+    let cursor_x = crate::input::place_cursor_x(inner.x, inner.width, prompt_w, s.input.cursor_col());
+    f.set_cursor_position((cursor_x, inner.y));
 }
 
 fn mode_label(m: SearchMode) -> &'static str {

crates/kebab-tui/tests/library.rs

@@ -249,6 +249,43 @@ fn filter_overlay_accepts_hangul_tags() {
     );
 }
 
+/// p9-fb-10: filter overlay calls f.set_cursor_position so ratatui
+/// shows the caret on the focused field. Pin: after opening the
+/// overlay, render → terminal cursor is set + has non-zero x
+/// (the label offset > 0).
+#[test]
+fn filter_overlay_render_places_cursor_on_focused_field() {
+    let mut app = app_with_docs(vec![make_doc("a.md", "A", vec![])]);
+    // Open filter.
+    let _ = kebab_tui::handle_key_library(
+        &mut app,
+        KeyEvent::new(KeyCode::Char('f'), KeyModifiers::NONE),
+    );
+    let backend = TestBackend::new(80, 20);
+    let mut terminal = Terminal::new(backend).unwrap();
+    terminal
+        .draw(|f| {
+            let area = Rect::new(0, 0, 80, 20);
+            render_library(f, area, &app);
+        })
+        .expect("render must not panic");
+    // After draw, ratatui calls backend.set_cursor_position when the
+    // frame's cursor_position is Some. The terminal's
+    // get_cursor_position proxies to the backend.
+    let pos = terminal.get_cursor_position().expect(
+        "filter overlay must call set_cursor_position, so cursor pos must be readable",
+    );
+    // The Tags label ("tags_any (csv): ") has display_width 16; inner.x
+    // is 1 (inside border). With empty input cursor_col=0, expected x=17.
+    // We assert x>0 to avoid hardcoding the exact layout geometry while
+    // still confirming set_cursor_position was called with a meaningful
+    // offset (not stuck at origin).
+    assert!(
+        pos.x > 0,
+        "cursor x should be positive (label offset > 0): {pos:?}"
+    );
+}
+
 /// p9-fb-10: Library renders Hangul / CJK titles without overflowing
 /// the title column. Smoke pin — render with a mixed Korean fixture
 /// and confirm no panic + the truncated width fits the column.