hide the status indicator when the answer stream starts (#4101)

This eliminates a "bounce" at the end of streaming where we hide the
status indicator at the end of the turn and the composer moves up two
lines.

Also, simplify streaming further by removing the HistorySink and
inverting control, and collapsing a few single-element structures.

codex-rs/tui/src/markdown_stream.rs

@@ -1,5 +1,3 @@
-use std::collections::VecDeque;
-
 use codex_core::config::Config;
 use ratatui::text::Line;
 
@@ -97,59 +95,6 @@ impl MarkdownStreamCollector {
     }
 }
 
-pub(crate) struct StepResult {
-    pub history: Vec<Line<'static>>, // lines to insert into history this step
-}
-
-/// Streams already-rendered rows into history while computing the newest K
-/// rows to show in a live overlay.
-pub(crate) struct AnimatedLineStreamer {
-    queue: VecDeque<Line<'static>>,
-}
-
-impl AnimatedLineStreamer {
-    pub fn new() -> Self {
-        Self {
-            queue: VecDeque::new(),
-        }
-    }
-
-    pub fn clear(&mut self) {
-        self.queue.clear();
-    }
-
-    pub fn enqueue(&mut self, lines: Vec<Line<'static>>) {
-        for l in lines {
-            self.queue.push_back(l);
-        }
-    }
-
-    pub fn step(&mut self) -> StepResult {
-        let mut history = Vec::new();
-        // Move exactly one per tick to animate gradual insertion.
-        let burst = if self.queue.is_empty() { 0 } else { 1 };
-        for _ in 0..burst {
-            if let Some(l) = self.queue.pop_front() {
-                history.push(l);
-            }
-        }
-
-        StepResult { history }
-    }
-
-    pub fn drain_all(&mut self) -> StepResult {
-        let mut history = Vec::new();
-        while let Some(l) = self.queue.pop_front() {
-            history.push(l);
-        }
-        StepResult { history }
-    }
-
-    pub fn is_idle(&self) -> bool {
-        self.queue.is_empty()
-    }
-}
-
 #[cfg(test)]
 pub(crate) fn simulate_stream_markdown_for_tests(
     deltas: &[&str],