[WIP] Fix sluggishness and overflow in import theme modal

[Copilot]

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This section details on the original issue you should resolve

<issue_title>Import-theme modal: paste editor re-parses on every keystroke + layout overflows on real tweakcn exports</issue_title>
<issue_description>## Symptom

The new tweakcn import modal (app/src/settings_view/import_theme_modal.rs) is sluggish and visually broken when a user pastes a real tweakcn export.

Visible in this screenshot during smoke: the paste box renders the CSS but the modal extends well beyond the viewport and typing/scrolling lags.

A real tweakcn export is ~80–100 lines, including @import "tailwindcss";, @custom-variant dark, font/radius/shadow vars, chart colors, sidebar primary/accent/border/ring — far more than the ~40-line synthetic fixture (crates/integration/tests/data/tweakcn_sample.css) the modal was sized against.

Root causes

1. No debounce on parse. handle_css_editor_event in app/src/settings_view/import_theme_modal.rs invokes on_css_changed on every EditorEvent::Edited:

```rust
fn handle_css_editor_event(&mut self, event: &EditorEvent, ctx: &mut ViewContext) {
if let EditorEvent::Edited(_) = event {
let text = self.css_editor.read(ctx, |editor, app| editor.buffer_text(app));
self.on_css_changed(text, ctx); // ← runs parse_blocks() on every keystroke / paste byte
}
}
```

For a paste, this fires once with the full content — but on subsequent edits (e.g., the user adjusting the slug, then editing the CSS again), every character triggers a full re-parse + UI notify.

2. Layout overflow. The modal's outer container doesn't appear to constrain the CSS editor's height or wrap it in a scroll view. Long content extends the modal past viewport bounds (visible in the screenshot — modal stretches to the bottom of the screen with no scroll affordance).

Suggested fix

• Debounce parsing. Add a ~200 ms timer (matching the spec at docs/superpowers/specs/2026-05-20-theme-customization-tweakcn-design.md:221). Either schedule via ctx (warpui has a timer pattern) or use the existing debounce helper if there is one.
• Constrain the CSS editor's height to e.g. 12–16 lines and wrap in a vertical scroll view. The single-line name input is fine as-is.
• Reduce per-render cost in render() — currently every parse result rebuilds the preview row. If preview generation is heavy, cache it.

Related

• Issue Theme save silently broken in default ./script/run builds (missing local_fs) #90 — local_fs default features bug; both surface during the same smoke flow.
• Spec section 5 ("Import UX") at docs/superpowers/specs/2026-05-20-theme-customization-tweakcn-design.md:215-234 explicitly called out the 200ms debounce; it didn't make it into the implementation.

Branch

Modal landed on castcodes/theme-tweakcn-impl commit d50ac78d.</issue_description>

Comments on the Issue (you are @copilot in this section)

[BunsDev]

Release scope note: deferred from v0.0.6. This is draft UI/import polish and should not block the OSS release unless Val explicitly retargets it.

[Copilot]

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[BunsDev]

Closing as superseded by #107. This branch is stale against current main and would remove the landed tweakcn import work instead of fixing the modal path.