WPF/Accessibility: Setting FontSize property prevents system font scaling from working

[yuyoyuppe]

Environment:

• .NET Core 3.1

Problem description:
While doing an accessibility pass for FancyZones Editor in PowerToys, we've encountered this issue. I've given a detailed explanation in this comment, but it boils down to the difference between FontSize property behavior in UWP/WPF.

Actual behavior:
Setting FontSize property in WPF prevents system-wide font scaling setting from working, no matter which context switch overrides are enabled.

Expected behavior:
Same as with UWP apps: elements' font size changes depending on the scale setting even when FontSize is explicitly set for them.

Minimal repro:

• create Blank WPF app
• go to MainWindow.xaml and add the following xaml inside the Grid:

<TextBlock Text="THE TEXT" FontSize="24"/>

• launch the application
• Start ->"Make text size bigger" -> Change to e.g. 200%
• Observe that the text size remains the same

Now repeat the steps with a blank UWP app and observe that "Make text size bigger" option affects text size.

[singhashish-wpf]

@yuyoyuppe "em" units are not supported as of now by WPF. For PowerToys, you can get the issue resolved by using the Markup-extension, as described here: https://stackoverflow.com/questions/653918/wpf-analogy-for-em-unit

[Daniellled]

I was wondering if this has had any movement?

Adding the following code to WinUI3

<Button x:Name="myButton" Click="myButton_Click" FontSize="12">Click Me</Button>

Appears to honor font scaling. Yet if I were to do the same with WPF the font would not scale.

It appears 3701 has the same problem but offers a different solution.