Custom Bounds, Resize, and Rotation

If the built-in behavior is close but not exact, ZoomBorder exposes virtual hooks for custom policy.

Extensibility Points

GetContentBounds(): return the effective bounds in child content coordinates; these bounds constrain panning and define scrollbar extent when BoundsMode="Custom"
ValidateTransform(Matrix newMatrix): accept or veto the final proposed transform
InvalidateContentBounds(): reapply constraints and refresh scrollbar state after dynamic custom bounds change
OnResized(Size oldSize, Size newSize): apply custom resize policy

Large Panning Area With Scrollbars

Custom uses a finite content-coordinate rectangle. Returning a rectangle larger than the child creates extra navigable space while keeping scrollbar positions meaningful:

public class CustomZoomBorder : ZoomBorder
{
    private Rect _navigationBounds = new(-5000, -5000, 10000, 10000);

    protected override Rect GetContentBounds() => _navigationBounds;

    protected override bool ValidateTransform(Matrix newMatrix)
    {
        return newMatrix.M11 is >= 0.1 and <= 20;
    }

    public void SetNavigationBounds(Rect bounds)
    {
        _navigationBounds = bounds;
        InvalidateContentBounds();
    }
}

<ScrollViewer HorizontalScrollBarVisibility="Auto"
              VerticalScrollBarVisibility="Auto">
  <local:CustomZoomBorder BoundsMode="Custom"
                          Stretch="None">
    <!-- content -->
  </local:CustomZoomBorder>
</ScrollViewer>

Panning remains bounded by the returned rectangle. Truly infinite panning cannot be represented by finite scrollbars; choose bounds large enough for the application’s workspace.

When To Override

content bounds depend on domain data rather than only the child's measured bounds
certain zoom or pan regions must be blocked
resize behavior needs to preserve a custom anchor or workflow-specific invariant

Rotation Considerations

Rotation state is configurable, but advanced rotation-heavy surfaces should validate how rotation interacts with bounds, serialization, and any custom overlay math before relying on it as a full scene-graph feature.