Custom Expander style requires BasedOn to be set

[kazo0]

Seems like an issue with winui: microsoft/microsoft-ui-xaml#6528

The suggestion in that issue is to remove the BasedOn. Curious that it is required for non-Windows platforms.

For now, you can use platform conditionals and define this in your xaml

xmlns:not_win="http://uno.ui/not_win"
mc:Ignorable="not_win"

and use this for BasedOn:

not_win:BasedOn="{StaticResource DefaultExpanderStyle}"

I will create an issue in the Uno repo to investigate the need for the BasedOn in Uno platforms

Originally posted by @kazo0 in #17774 (reply in thread)

If you remove the BasedOn="{StaticResource DefaultExpanderStyle}" from the custom Expander style in the attached sample app, it renders properly on Windows but does not display properly on other platforms

ReproWinStyle.zipWithout BasedOn (Windows):

Without BasedOn (Android):

With BasedOn (Android):

[jeromelaban]

@kazo0 is this a blocker?

[kazo0]

@jeromelaban it's not blocking anything, just something we noticed

[Xiaoy312]

counter-intuitively, it seems that BasedOn is to be expected?

For system controls that use WinUI styles (Windows.UI.Xaml.Controls namespace), set BasedOn="{StaticResource Default<ControlName>Style}", where <ControlName> is the name of the control. [...]
For WinUI 2 controls (Microsoft.UI.Xaml.Controls namespace), the default style is defined in the metadata, so omit BasedOn.
-- https://learn.microsoft.com/en-us/windows/apps/design/style/xaml-styles#avoid-restyling-controls

^ even MS is suggesting that