[master-wayne7]

In ExpansionTileTransitionMode.fade (the default), CupertinoExpansionTile draws the fade with an OverlayPortal clone of the child, while the in-flow child is kept at zero opacity during the animation. The overlay was only shown from _onHeaderTap, so expanding or collapsing through an ExpansibleController (controller.expand() / controller.collapse()) never showed it: the child popped in at the end of expansion and disappeared at the start of collapse.

This PR makes the tile listen to its ExpansibleController and show the fade overlay on every expansion change, whatever triggered it. The listener is moved over when the controller is swapped in didUpdateWidget and removed in dispose. The header-tap path no longer shows the overlay itself, since the listener now covers it.

Tests:

• Added Fades the child when toggled by ExpansibleController, which checks that the clone is mid-fade halfway through both expansion and collapse.
• Updated two existing controller tests that expected one Content widget after controller.expand(). Header taps already produce two (the animating child plus the fading clone), so these expectations reflected the bug.

Fixes flutter/flutter#194264

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Code Review

This pull request modifies CupertinoExpansionTile to listen to its controller's expansion state changes and trigger the fade transition, ensuring the transition occurs when toggled via an ExpansibleController. It also updates existing tests and adds a new test to verify this behavior. The review feedback suggests guarding the _fadeController.show() call in _handleExpansionChanged to only run when widget.transitionMode is ExpansionTileTransitionMode.fade to avoid redundant calls in other transition modes.