fix(ui): standardise Agent Stream scroll to match Events panel

- Change agent_scroll_down to no-op when auto-scrolling (None), matching
  events_scroll_down and tools_scroll_down behavior
- Compute proper max_offset for agent mouse scroll using inner height
  from rect instead of raw agent_line_count
- Remove leftover .min(80) cap on tools scroll range from pre-.rev()
  removal era

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

Author: d4ncer

src/ui/app.rs

@@ -478,10 +478,12 @@ fn process_mouse(
 
     if let Some(ref r) = areas.agent {
         if in_rect(r) {
+            let inner_h = r.height.saturating_sub(2) as usize;
+            let max_offset = state.agent_line_count.saturating_sub(inner_h);
             if scroll_lines < 0 {
                 state.agent_scroll_up((-scroll_lines) as usize);
             } else {
-                state.agent_scroll_down(scroll_lines as usize, state.agent_line_count);
+                state.agent_scroll_down(scroll_lines as usize, max_offset);
             }
             return;
         }
@@ -504,8 +506,7 @@ fn process_mouse(
     if let Some(ref r) = areas.tools {
         if in_rect(r) {
             let inner_h = r.height.saturating_sub(2) as usize;
-            let total = state.tools.len().min(80);
-            let max_offset = total.saturating_sub(inner_h);
+            let max_offset = state.tools.len().saturating_sub(inner_h);
             if scroll_lines < 0 {
                 state.tools_scroll_up((-scroll_lines) as usize);
             } else {

src/ui/state.rs

@@ -235,16 +235,17 @@ impl AppState {
     }
 
     /// Scroll the Agent Stream panel down by `n` lines, capped at the bottom.
-    /// Passing the total line count allows clamping.
+    /// Reaching the bottom resumes auto-scroll.
     pub fn agent_scroll_down(&mut self, n: usize, max_offset: usize) {
-        let current = self.agent_scroll.unwrap_or(max_offset);
-        let new = (current + n).min(max_offset);
-        // If we've scrolled back to the bottom, resume auto-scroll.
-        if new >= max_offset {
-            self.agent_scroll = None;
-        } else {
-            self.agent_scroll = Some(new);
+        if let Some(offset) = self.agent_scroll {
+            let new = (offset + n).min(max_offset);
+            if new >= max_offset {
+                self.agent_scroll = None;
+            } else {
+                self.agent_scroll = Some(new);
+            }
         }
+        // If None (auto-scroll), down is a no-op — already at bottom.
     }
 
     /// Reset Agent Stream to auto-scroll (follow the tail).