ScrollableControl.ScaleDockPadding seems to be broken?

[hughbe]

Problem:

Looking at the implementation of `ScrollableControl.Scale{Core}:

internal void ScaleDockPadding(float dx, float dy)
{
    dockPadding?.Scale(dx, dy);
}

[EditorBrowsable(EditorBrowsableState.Never)]
protected override void ScaleCore(float dx, float dy)
{
    ScaleDockPadding(dx, dy);
    base.ScaleCore(dx, dy);
}

Where DockPadding.Scale is implemented as

internal void Scale(float dx, float dy) => _owner.Padding.Scale(dx, dy);

I would expect the scrollable control's padding to scale by dx and dy. However, the following tests demonstrate that no scaling is happening.

I think something may be weird in that calling Padding.Scale does not seem to affect the underlying Padding instance stored in Control

[WinFormsFact]
public void ScrollableControl_Scale_InvokeWithoutPaddingWithDockPadding_Success()
{
    using var control = new ScrollableControl();
    Assert.Equal(0, control.DockPadding.Left);
    Assert.Equal(0, control.DockPadding.Top);
    Assert.Equal(0, control.DockPadding.Right);
    Assert.Equal(0, control.DockPadding.Bottom);
    control.Scale(10, 20);

    Assert.Equal(0, control.DockPadding.Left);
    Assert.Equal(0, control.DockPadding.Top);
    Assert.Equal(0, control.DockPadding.Right);
    Assert.Equal(0, control.DockPadding.Bottom);
    Assert.Equal(Padding.Empty, control.Padding);
}

[WinFormsFact]
public void ScrollableControl_Scale_InvokeWithoutPaddingWithoutDockPadding_Success()
{
    using var control = new ScrollableControl();
    control.Scale(10, 20);
    Assert.Equal(0, control.DockPadding.Left);
    Assert.Equal(0, control.DockPadding.Top);
    Assert.Equal(0, control.DockPadding.Right);
    Assert.Equal(0, control.DockPadding.Bottom);
    Assert.Equal(Padding.Empty, control.Padding);
}

[WinFormsFact]
public void ScrollableControl_Scale_InvokeWithPaddingWithDockPadding_Success()
{
    using var control = new ScrollableControl
    {
        Padding = new Padding(1, 2, 3, 4)
    };
    control.Scale(10, 20);
    Assert.Equal(1, control.DockPadding.Left);
    Assert.Equal(2, control.DockPadding.Top);
    Assert.Equal(3, control.DockPadding.Right);
    Assert.Equal(4, control.DockPadding.Bottom);
    Assert.Equal(new Padding(1, 2, 3, 4), control.Padding);
}

[WinFormsFact]
public void ScrollableControl_Scale_InvokeWithPaddingWithoutDockPadding_Success()
{
    using var control = new ScrollableControl
    {
        Padding = new Padding(1, 2, 3, 4)
    };
    Assert.Equal(1, control.DockPadding.Left);
    Assert.Equal(2, control.DockPadding.Top);
    Assert.Equal(3, control.DockPadding.Right);
    Assert.Equal(4, control.DockPadding.Bottom);
    control.Scale(10, 20);
    Assert.Equal(1, control.DockPadding.Left);
    Assert.Equal(2, control.DockPadding.Top);
    Assert.Equal(3, control.DockPadding.Right);
    Assert.Equal(4, control.DockPadding.Bottom);
    Assert.Equal(new Padding(1, 2, 3, 4), control.Padding);
}

[weltkante]

Its because its a struct, _owner.Padding.Scale(dx, dy); will return a copy of the owners Padding from the _owner.Padding property, so Scale(dx, dy) will operate on a copy of the original struct which will be immediately discarded.

Looking at reference source this seems to be broken in Desktop at as well, might have never been working. If you fix it you have to make sure any callers within WinForms can deal with the change (they might have had their own workarounds - or maybe the whole infrastructure never worked?)