* Make FlyoutBase.IsOpen a public StyledProperty with two-way binding support
Convert IsOpen from a DirectProperty with a protected setter to a
StyledProperty with a public setter and TwoWay default binding mode.
This enables MVVM scenarios where a ViewModel can control flyout
visibility through data binding.
The implementation mirrors the established Popup.IsOpen pattern:
- Reentrancy guard (BeginIgnoringIsOpen scope) prevents recursive
property change notifications when internal code syncs the property
- SetCurrentValue preserves active bindings and styles (enforced by
analyzer AVP1012)
- _isOpen field tracks actual open state independently of the property
value, since the property system sets the value before the change
handler fires
- _lastPlacementTarget enables re-opening at the last known target
when IsOpen is set to true via binding
- IsOpen reverts to false when no target is available or opening is
cancelled, and reverts to true when closing is cancelled, keeping
the property honest
Fixes#18716
* ci: retrigger checks
* Add API suppression for FlyoutBase.IsOpenProperty type change
Suppress CP0002 for the intentional binary breaking change from
DirectProperty<FlyoutBase, bool> to StyledProperty<bool>.
* Pre-register owning control as flyout placement target
When Button.Flyout or SplitButton.Flyout is set, the owning control
now registers itself as the default placement target via an internal
SetDefaultPlacementTarget method. This allows IsOpen = true to work
on first use without a prior ShowAt call, addressing review feedback
from MrJul.
* Remove TwoWay default binding mode from IsOpenProperty
Follow Avalonia convention: Popup.IsOpen and ToolTip.IsOpen use the
default OneWay binding mode. TwoWay is reserved for input controls.
Users opt in with Mode=TwoWay when needed.
Nathan Nguyen