fix: reset popup scroll state synchronously on new prompt (#83)

[StanMarek]

Summary

Fixes #83 — completion popup occasionally renders on top of the prompt after Ctrl+C on Terminal.app and Ghostty.

Root cause

InputHandler::scroll_deficit (crates/gc-pty/src/handler.rs:189) was only reset via the async CPR roundtrip drained inside trigger() (handler.rs:614). After Ctrl+C, the shell prints ^C\n + new prompt + OSC 7771;A + OSC 7770("") in one batched read. Task B writes CSI 6n and immediately calls trigger() with buffer_dirty=true — but the terminal's CPR response is still in flight on stdin. take_cpr_synced() returns false, scroll_deficit stays at its pre-Ctrl+C value, and dismiss() (handler.rs:1050) didn't reset it either.

If the user types within that window, final_cursor_row = real_row − scroll_deficit produces start_row = real_row — the popup anchor lands on the new prompt row. With scroll_deficit=1 you get exactly the symptom from the issue: (b<popup>Book-Air GitHub % su with the popup overlaid at column 2.

Both terminals are affected because the race lives in the proxy/handler — not in DECSET 2026 vs pre-render-buffer.

Fix

Add a synchronous prompt-boundary signal independent of CPR. Drain it at every render entry so a re-render that fires before the CPR roundtrip lands still sees a clean per-prompt state.

• gc-parser/src/state.rs — new prompt_started: bool field on TerminalState. Public take_prompt_started() mirrors take_cpr_synced (atomic drain). pub(crate) mark_prompt_started() setter.
• gc-parser/src/performer.rs — call mark_prompt_started() inside both OSC 133;A and OSC 7771;A handlers, alongside the existing request_cursor_sync / set_prompt_row calls.
• gc-pty/src/handler.rs:
  • New drain_prompt_started_reset helper called at the start of render, try_merge_dynamic, navigation in process_key_visible, and accept_with_chaining. On true: scroll_deficit = 0 and last_layout = None.
  • trigger() drains both flags. prompt_started resets both fields; cpr_synced (fallback) only resets scroll_deficit because a CPR sync within the same prompt context shouldn't blow away a valid layout.
  • dismiss() now resets scroll_deficit = 0.
  • Doc-comment on scroll_deficit updated to reflect all reset paths.

CPR-sync reset retained as defense-in-depth.

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test --workspace — 942 passed, 0 failed
• 8 new tests (4 parser, 4 handler) covering the regression:
  • prompt_started_flag_default_false
  • prompt_started_set_then_drained
  • test_osc133_prompt_a_sets_prompt_started
  • test_osc7771_prompt_a_sets_prompt_started
  • test_render_resets_scroll_deficit_on_prompt_started
  • test_dismiss_resets_scroll_deficit
  • test_try_merge_dynamic_resets_scroll_deficit_on_prompt_started
  • test_trigger_resets_last_layout_on_prompt_started_only
• Manual repro: 24-row Terminal.app, run a command that surfaces history, hit Ctrl+C while popup is visible, type immediately. Confirm popup no longer overlays the new prompt.
• Same on Ghostty.

[StanMarek]

Old PR