[nicoburns]

Summary

Since #5954 (ratatui 0.29 → 0.30), the dx serve TUI stalls for ~2s on every log line and then silently drops it. Symptoms: the dashboard looks hung in odd places ("Installing tooling" / "Compiling n/m" for 30s+ with nothing printed), and t/v only seem to work sometimes - especially not during a build with --trace on.

Cause: in ratatui 0.30, Terminal::clear() queries the cursor position (ESC[6n), and insert_before - which render_log called once per log line - ends with clear(). The cursor query needs crossterm's input-reader lock, which the EventStream background thread holds while it is parked waiting for a key. So the query waits out its 2s timeout, errors, and the log line is lost. The lock is only free right after a keypress, hence the intermittent feel: a t pressed mid-build lands only once the queued cargo output has been "rendered" (2s per line), and with --trace that queue is ~1000 lines.

Fix:

• Enable ratatui's scrolling-regions feature. Its insert_before sets a VT scroll region covering the rows above the viewport, scrolls that region, and draws the lines into the gap - it never clears the frame or queries the cursor (and the frame no longer flickers/redraws per log).
• That makes the manual cursor/padding hack in render_log and serve/ansi_buffer.rs unnecessary: logs are built as styled Lines, word-wrapped with Paragraph, and inserted with a single insert_before per render. This is the "future" the old FIXME comments in render_log described.
• TraceController::try_recv + a drain in the serve loop so an already-queued burst of trace messages is rendered in one pass instead of one redraw per line, keeping keyboard input responsive.
• Drop the now-unused unicode-segmentation dependency.

Net: +54 / -232.

Test plan

Tested on macOS with a release build of dioxus-components (dx serve --release -p preview --features native --macos), which previously reproduced this reliably:

• --trace from the start: ~1000 cargo lines render within the same second (before: 2s each / dropped).
• r then t ×2 mid-build: each toggle handled the instant it was pressed (before: ~15s late).
• v, / (viewport resize) work mid-build; frame intact; logs present in scrollback; app launches.
• cargo clippy -p dioxus-cli --all-targets clean; cargo test -p dioxus-cli --bin dx 162/162.

Note: scroll-region insert_before relies on the terminal pushing lines scrolled off a top-anchored region into scrollback. That is standard (iTerm2, Terminal.app, tmux, kitty, Alacritty, WezTerm, VTE, Windows Terminal) but reviewers on other terminals may want to sanity-check dx serve --trace scrollback.