Uno Platform ComboBox Multi-Window Bug Sample

This is a minimal reproduction sample for a bug in Uno Platform 6.4.26 where ComboBox controls become unresponsive after specific multi-window interactions on macOS Skia.

What This Demonstrates

This sample reproduces a bug where:

1. Opening a secondary window
2. Double-clicking an item in that window (which sends a message to the main window)
3. Returning to the main window

...causes ALL ComboBox controls to stop opening when clicked.

Quick Start

Prerequisites

• .NET 9.0 SDK
• macOS (where the bug reproduces)
• Uno Platform workload installed

Build and Run

# From this directory
dotnet build -f net9.0-desktop
dotnet run -f net9.0-desktop

Reproduction Steps

1. Launch the app - you'll see the main window with instructions
2. Click the ComboBox - it should open normally ✅
3. Click "Open Secondary Window"
4. Double-click any item in the TreeView of the secondary window
5. Return to main window - it automatically activates
6. Click the ComboBox again - ❌ BUG: It won't open!
7. Alt-Tab away and back - ComboBox works again ✅

What to Observe

The sample includes diagnostic counters and a live event log showing:

• PointerPressed events - These WILL fire when clicking the broken ComboBox
• DropDownOpened events - These will NOT fire (the bug)
• DropDownClosed events - For tracking dropdown state

When the bug is active, you'll see PointerPressed events firing but no corresponding DropDownOpened events.

Project Structure

UnoComboBoxBugSample/
├── UnoComboBoxBugSample.csproj  # Project file (Uno.Sdk 6.4.26)
├── Program.cs                    # Entry point
├── App.xaml / App.xaml.cs        # Application entry point
├── MainWindow.xaml               # Main window with ComboBox + diagnostics
├── MainWindow.xaml.cs            # Event handling and logging
├── SecondaryWindow.xaml          # Secondary window with TreeView
├── SecondaryWindow.xaml.cs       # Double-click handler
└── README.md                     # This file

Real-World Context

This bug was discovered in a production mapping application where:

• The main window has a map view with a ComboBox for selecting map types
• A secondary window shows a TreeView of routes/waypoints
• Double-clicking a route sends messages to navigate the map
• After this interaction, the map picker ComboBox stops working

The pattern of "secondary window → user interaction → message to main window → window close" triggers the bug consistently.

Platform Specificity

This bug appears to be macOS Skia-specific. Expected behavior on other platforms:

• ✅ Windows (WinUI3) - ComboBox should work
• ✅ iOS - ComboBox should work
• ✅ Android - ComboBox should work
• ? WebAssembly - Untested

Technical Details

Suspected Issue: Uno Platform's multi-window focus/pointer handling on macOS Skia has a bug where:

• Secondary window activation + message passing + window closure
• Leaves the main window in a corrupted pointer/focus state
• ComboBox controls receive PointerPressed but the dropdown mechanism is blocked
• Alt-tabbing resets the state

Uno SDK Version: 6.4.26 Target Framework: net9.0-desktop .NET Version: 9.0

Upstream Issue

• Status: To be filed with Uno Platform
• Repository: https://github.com/unoplatform/uno
• Issue Number: [To be added after filing]

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This minimal sample was created to help the Uno Platform team identify and fix the underlying multi-window focus bug.