perf(ui): reduce rebuilds and virtualize heavy surfaces

[ZhuchkaTriplesix]

Context

UI performance review across lib/. Goal: cut unnecessary widget rebuilds and virtualize the heaviest surfaces (result grid, connection tree, code editor). Findings are grouped by severity; each can be tackled incrementally.

Review-only for now — no code changed yet. This issue tracks the implementation.

HIGH

H1. Interface scale rebuilds the whole app on every slider tick

• lib/app/app.dart — single ListenableBuilder merges themeController + uiScaleController; setScalePreview() fires notifyListeners() per drag step (lib/core/layout/ui_scale_controller.dart:18-23), rebuilding ShadcnApp → MainScreen → tree → SQL workspaces → editors.
• materialThemeFromQuerya() / lightShadcnTheme / darkShadcnTheme / activeTheme reallocate ThemeData (VS Code color merge + applyTokenColorsToEditor) 3x per frame even though only scale changed.
• Fix: local ValueNotifier<double> for live preview in the preferences dialog; call commitScale() only on onChangeEnd. Split theme-only vs scale-only builders. Memoize ThemeData/QueryaTheme by (preset, overrides, brightness).

H2. Result grid has no virtualization

• lib/features/main_screen/results_tab.dart:155-198 — material.Table + IntrinsicColumnWidth + ...rows.map(... SelectableText ...) materializes all cells (cap = 5000 rows). Intrinsic pass is O(rows×cols).
• Fix: virtualized grid (ListView.builder + fixed/measured-once column widths), Text + "copy cell" instead of SelectableText, pagination. Wrap body in RepaintBoundary.

H3. Connection tree built eagerly, per-row TextPainter

• lib/features/connections/connections_panel.dart (SliverToBoxAdapter + Column), connections_panel_pg_tree.dart:957-983 / connections_panel_mysql.dart:447-510 (for loops build all tables/views at once).
• connections_panel_pg_tree.dart:40-48 — TextPainter.layout per row per build for overflow detection.
• Fix: flatten visible nodes → SliverList.builder; replace manual TextPainter with Text(overflow: ellipsis) + Tooltip; add ValueKey on leaf rows.

H4. Syntax highlighting runs synchronously per keystroke

• lib/core/editor/querya_highlight_controller.dart — highlighter.highlight(text) runs inside buildTextSpan for buffers < 8KB on every keystroke; no debounce.
• lib/core/editor/syntax_highlight_isolate.dart:126-129 — Future.value(syntaxHighlightInIsolate(job)) runs synchronously on the UI thread (no isolate despite the name).
• Fix: debounce (50-150ms) + always compute/isolate; reuse previous span while pending.

MEDIUM

• M1. Splitter drag setState rebuilds both panels/editor/grid — main_screen.dart:255-271, workspace_panel.dart:224-231, postgres_sql_workspace.dart:434-441, mysql_sql_workspace.dart. Fix: ValueNotifier<double> + narrow ValueListenableBuilder.
• M2. AppSettingsRevision.bump() on any setting write (incl. scale commit) reloads all open SQL workspaces — app_settings.dart:222-228, postgres_sql_workspace.dart:92-154. Fix: granular channels / exclude ui_scale.
• M3. ConnectionsPanel rebuilds on any MainScreenWorkspaceState change though it only needs activeConnection?.id — main_screen.dart:234-251.
• M4. Connection forms setState(() {}) per keystroke — postgresql_connection_form.dart:62, mysql_connection_form.dart:62, redis_connection_form.dart:53, mongodb_connection_form.dart:93. Fix: ValueNotifier<bool> formValid around buttons only.
• M5. Stats polling rebuilds full page — redis_view.dart:130-137, postgres_stats_view.dart:124-130, mongo_stats_view.dart:150-167. Fix: compare snapshot, split sections.
• M6. Redis key editor builds all hash/list/set members in Column inside SingleChildScrollView — redis_key_editor.dart:495-687.
• M7. Mongo documents: JsonEncoder.withIndent in build when expanded + hover setState + missing keys — mongo_documents_view.dart:220-237,404-481.
• M8. MySQL result row→string conversion is synchronous on UI thread; PG already uses compute(convertResultRowsToStrings) — mysql_sql_workspace.dart:155-166.
• M9. Theme/highlighter pair recreated on theme change without caching — syntax_highlight_service.dart, token_colors_highlighter_config.dart, theme_controller.dart:73-79.

LOW

• L1. RegExp compiled in build — sql_query_history_dialog.dart:72-75, postgres_stats_view.dart:555-557. Hoist to static final.
• L2. QueryaDropdown.menuChildren rebuilt every build — querya_dropdown.dart:201.
• L3. Hover setState in list rows (cumulative) — mongo/redis views.
• L4. TextEditingController not disposed in Redis TTL dialog — redis_key_editor.dart:694-709.

Suggested order

1. H1 (scale/theme decoupling) — biggest win, smallest change.
2. H4 (highlight debounce + true isolate).
3. H2 (result grid virtualization).
4. H3 (connection tree virtualization).
5. MEDIUM batch (splitter drag, settings revision, form validation).
6. LOW cleanups.