DataGrid with Fluent theme and lots of columns is 10x slower removing from view

Describe the bug

When the Fluent theme is used, a DataGrid with lots of columns (20+) is very slow when removed from the UI. (about 20x slower than adding) Using the base Light theme is about 70x faster when removing the control.

To Reproduce

1. Clone https://github.com/garyhertel/AvaloniaDataGridPerfTest.git
2. Start App
3. Click “Toggle” button to toggle DataGrid on and off
4. Look at the Debug output for times
5. You can switch the theme in the App.xaml

A few notes:

• Hardcoding or resizing the MainWindow Width cuts the performance hit by 10x, but this is still significantly slower than the base theme. My app also relies on the DataGrid autosizing itself so I’m not sure this can be permanently hard coded.
• I’m using a widescreen monitor so the default window width is ~2800 pixels

Expected behavior

Using the Fluent theme and lots of DataGrid columns, the DataGrid removal should match the performance of the base light theme.

Alternatively, some workaround to speed up removal would help. (although other people will probably run into this problem too)

Screenshots

Desktop (please complete the following information):

• OS: Windows 10 Pro
• Version: Avalonia 0.10.15

Additional context

Timings for the sample app using using different themes:

Width=2864

Light Theme, resized and half of columns showing

DataGrid added, [time = 52ms ]
DataGrid removed, [time = 56ms ]

DataGrid added, [time = 0ms ]
DataGrid removed, [time = 40ms ]

DataGrid added, [time = 0ms ]
DataGrid removed, [time = 71ms ]

Light Theme, all columns showing

DataGrid added, [time = 57ms ]
DataGrid removed, [time = 152ms ]

DataGrid added, [time = 0ms ]
DataGrid removed, [time = 137ms ]

DataGrid added, [time = 0ms ]
DataGrid removed, [time = 134ms ]

Fluent Light, resized and half of columns showing

DataGrid added, [time = 58ms ]
DataGrid removed, [time = 669ms ]

DataGrid added, [time = 0ms ]
DataGrid removed, [time = 662ms ]

DataGrid added, [time = 0ms ]
DataGrid removed, [time = 661ms ]

Fluent Light, all columns showing

DataGrid added, [time = 57ms ]
DataGrid removed, [time = 9876ms ]

DataGrid added, [time = 0ms ]
DataGrid removed, [time = 9788ms ]

DataGrid added, [time = 0ms ]
DataGrid removed, [time = 9789ms ]

[grokys]

Seems that the reason the above method is showing up so prominently in profiling is that the Window.DottedLineFocusAdorner :is(Control) style is adding thousands of handlers to Classes.CollectionChanged:

There are nearly 8000 class listeners being added to the window’s Classes collection:

[garyhertel]

Thanks for all the quick fixes everyone! This was the biggest blocker for the Fluent upgrade, so hopefully I can get that updated soon!