fix(claudbg-4gtn): clamp Space/PageDown scroll to end of transcript content

Compute total rendered lines on each page-down keypress and cap the
scroll offset so it cannot scroll past the last line.

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

.beans/claudbg-4gtn--tui-spacepagedown-scrolls-past-end-of-transcript.md

@@ -1,10 +1,15 @@
 ---
 # claudbg-4gtn
 title: 'TUI: Space/PageDown scrolls past end of transcript'
-status: todo
+status: completed
 type: bug
+priority: normal
 created_at: 2026-03-31T23:45:06Z
-updated_at: 2026-03-31T23:45:06Z
+updated_at: 2026-04-01T05:51:55Z
 ---
 
 Space/PageDown in the transcript view continues scrolling past the last line. It should stop at the bottom of the transcript content.
+
+## Summary of Changes
+
+In `src/tui/screens/transcript.rs`, the PageDown/Space handler now clamps scroll to the actual content length: computes `total_lines = build_chat_lines(...).len()` and `max_scroll = total_lines.saturating_sub(page_height)`, then applies `.min(max_scroll)` to prevent scrolling into empty space. Updated 3 existing tests to populate entries before testing scroll behavior.

src/tui/screens/transcript.rs

@@ -624,7 +624,10 @@ pub fn handle_transcript_event(event: Event, state: &mut AppState) -> bool {
         }
         KeyCode::PageDown | KeyCode::Char(' ') => {
             let page = (state.transcript_page_height as usize).max(1);
-            state.transcript_scroll = state.transcript_scroll.saturating_add(page);
+            let total_lines =
+                build_chat_lines(&state.transcript_entries, state.color_enabled).len();
+            let max_scroll = total_lines.saturating_sub(state.transcript_page_height as usize);
+            state.transcript_scroll = state.transcript_scroll.saturating_add(page).min(max_scroll);
             true
         }
         // Horizontal scroll — only meaningful in ChatLog.
@@ -1197,6 +1200,10 @@ mod tests {
     #[test]
     fn space_scrolls_down_by_page() {
         let mut state = transcript_state();
+        // Populate enough entries that clamping won't prevent scrolling by the full page.
+        state.transcript_entries = (0..100)
+            .map(|i| user_text_entry(&format!("line {i}")))
+            .collect();
         state.transcript_page_height = 20;
         handle_transcript_event(press(KeyCode::Char(' ')), &mut state);
         assert_eq!(state.transcript_scroll, 20);
@@ -1205,6 +1212,10 @@ mod tests {
     #[test]
     fn pagedown_scrolls_down_by_page() {
         let mut state = transcript_state();
+        // Populate enough entries that clamping won't prevent scrolling by the full page.
+        state.transcript_entries = (0..100)
+            .map(|i| user_text_entry(&format!("line {i}")))
+            .collect();
         state.transcript_page_height = 15;
         handle_transcript_event(press(KeyCode::PageDown), &mut state);
         assert_eq!(state.transcript_scroll, 15);
@@ -1392,6 +1403,8 @@ mod tests {
     #[test]
     fn page_scroll_uses_one_when_height_is_zero() {
         let mut state = transcript_state();
+        // Need at least one line of content so clamping allows scroll=1.
+        state.transcript_entries = vec![user_text_entry("hello")];
         state.transcript_page_height = 0;
         handle_transcript_event(press(KeyCode::Char(' ')), &mut state);
         assert_eq!(state.transcript_scroll, 1);