Add useIndexedTab property for smooth tab switching with lazy-loaded indexed building

[usmanvrtx]

Description:
Introduces useIndexedTab, a boolean property that enables IndexedStack-based tab building in TabBar widgets.

Problem it solves:
By default, when switching tabs, the previous tab's widget is removed from the widget tree. Switching back to that tab requires rebuilding it, which causes lag or freezing if the tab contains heavy data.

Solution:
With useIndexedTab: true, tabs are built on-demand and cached. When you switch to a tab for the first time, it's built and cached. Tab switching becomes instant—no rebuild, no delay, no freeze. Unvisited tabs remain lazy and are not built until accessed.

How it works:

• Tabs are built only when first accessed (lazy loading)
• Built tabs are cached and kept in the widget tree while the screen is active
• Uses IndexedStack (expanded) or Column with Offstage (non-expanded) to maintain widgets in tree
• Only the active tab is visible; others remain cached and ready

Impact:
Eliminates freezed transitions when switching between tabs with heavy content.

Screen.Recording.2026-04-28.at.01.10.53.mov

[Copilot]

Pull request overview

Adds an opt-in useIndexedTab flag to TabBar widgets to keep visited tab bodies alive (built lazily and cached) to avoid lag/freezes when switching back to heavy tabs.

Changes:

• Added useIndexedTab property plumbing on BaseTabBar / TabBarController.
• Implemented lazy, cached tab-body rendering via IndexedStack (expanded) or Column + Offstage (non-expanded).
• Added a selectedIndex bounds fix during TabController reinitialization for conditional tab visibility.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
modules/ensemble/lib/layout/tab_bar.dart	Adds useIndexedTab setter, tab-body caching, and controller reinit bounds guard.
modules/ensemble/lib/layout/tab/tab_bar_controller.dart	Adds useIndexedTab field to the controller (plus minor formatting change).

Comments suppressed due to low confidence (1)

modules/ensemble/lib/layout/tab_bar.dart:127

• _getValidInitialIndex() clamps the index used for TabController.initialIndex, but it does not update widget._controller.selectedIndex itself. If selectedIndex is configured (or data-bound) to a value >= items.length, the controller still holds the invalid value and buildSelectedTab() / _buildTabBodies() will index into items[selectedIndex] and crash. Consider clamping and writing back selectedIndex to the computed safe value when initializing/reinitializing the controller.

  void _initializeTabController() {
    final safeIndex = _getValidInitialIndex();
    if (widget.controller.useIndexedTab) {
      _cache = List<Widget?>.filled(widget.controller.items.length, null);
    }
    tabController = TabController(
      initialIndex: safeIndex,
      length: widget.controller.items.length,
      vsync: this,
    );
    tabController.addListener(notifyListener);
  }

  int _getValidInitialIndex() {
    // ensure the selectedIndex is valid, otherwise reset
    return widget._controller.selectedIndex < widget.controller.items.length
        ? widget._controller.selectedIndex
        : 0;
  }

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[TheNoumanDev]

LGTM

[TheNoumanDev]

LGTM