ColumnDefinition doesn't expand until resize

Question

Hi, I have a grid where one columnwidth is defined as *. The other columns are defined as Auto. The column with the *-definition contains a usercontrol derived from Panel that also implements IScrollInfo. During this control's MeasureOverride visibility is set to visible on a RepeatButton in another column (the RepeatButton's visibility is otherwise set to collapsed).

This does not cause the column to expand. This will only occur when I resize my window. A simplified example:

<DockPanel LastChildFill="True">
    <Grid DockPanel.Dock="Left">
        <Grid.ColumnDefinitions>
            <ColumnDefinition x:Name="TabItemColumn"/>
            <ColumnDefinition x:Name="ScrollRightColumn" Width="Auto"/>
        </Grid.ColumnDefinitions>
        <ScrollViewer x:Name="PART_ScrollViewer" 
                Grid.Column="0" 
                Margin="-1,0,0,0" 
                Height="32" 
                CanContentScroll="True" 
                VerticalScrollBarVisibility="Hidden" 
                HorizontalScrollBarVisibility="Hidden" 
                HorizontalAlignment="Left">
            <local:TabPanel 
                    x:Name="tabPanel" 
                    HorizontalAlignment="Left" 
                    IsItemsHost="True" />
        </ScrollViewer>

        <RepeatButton Style="{StaticResource RepeatButtonScrollRight}"
            Visibility="{Binding ElementName=tabPanel, Path=CanScrollRight, Converter={StaticResource _localBooleanConverter}}"
            Grid.Column="1">
        </RepeatButton>

The visibility of the RepeatButton is triggered correctly, and as far as I can tell it is actually rendered, but the ActualWidth of the containing column is zero until resize.

Any ideas?

Answer 1

You'll need to trigger a layout change after changing the column width. Use InvalidateArrange() on the proper parent element. Take care to avoid infinite cycles.

Answer 2

InvalidateArrange is a good answer, but unfortunately it is silently ignored during the time a control is actually being arranged. So the trick is to call it after the arrange is complete.

This may work (I haven't tried it):

Dispatcher.BeginInvoke(DispatcherPriority.Normal, new Action(() =>
{
  grid.InvalidateArrange();
}));

If this doesn't work, you might try calling InvalidateArrange and/or InvalidateMeasure on the RepeatButton, also within a Dispatcher.BeginInvoke callback.