App.xaml

<Application x:Class="VrcPhotoManager.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:VrcPhotoManager"
             StartupUri="MainWindow.xaml" ShutdownMode="OnMainWindowClose">
    <Application.Resources>
        <!-- Applies to every plain ToolTip="..." string in the app (WPF's default ToolTip
             doesn't wrap text - it just grows as wide as the string needs, which for longer
             explanatory tooltips ran clear across the screen on one line). {Binding} in the
             template binds to the ToolTip's Content itself (the string), which is the standard
             way to reach it since a plain-string tooltip has no separate DataContext. -->
        <Style TargetType="ToolTip">
            <Setter Property="MaxWidth" Value="320" />
            <Setter Property="ContentTemplate">
                <Setter.Value>
                    <DataTemplate>
                        <TextBlock Text="{Binding}" TextWrapping="Wrap" />
                    </DataTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Application.Resources>
</Application>