Autoscroll problem on mousedown

Question

I have a custom control inside an autoscroll panel. When the user control extends beyond the width of the panel, the scroll bars appear. When you then scroll any distance, and mousedown on the control, the scrollbar snaps back to 0. Does any one know why that might be? I'm pretty sure I am not trying to change the value of the scrollbar anywhere...

Thanks

EDIT: This only appears to happen once, the first time you click on it, every other time it works as expected

EDIT 2: It also happens when you bring a new window up, and then go back to the C# window

Answer 1

If you have a control (like a TextBox) that is much wider than it's container and you scroll to it's end, then click the control, you will be scrolled back to the Location of the control.

The clicked control gains focus and the scrolling occurs automatically, that is standard behaviour of winforms.

If you want to negate that, you will have to intercept SetAutoScrollPosition of the container (ScrollableControl) or use another mechanism to revert to the original position.

If the control already has focus and you then scroll, clicking it again won't change the AutoScrollPosition of the container.

Answer 2

I worked this out for VB.net. To try this create a WinForms project and:

1. Place a Panel1 on Form1 and a TextBox1 inside Panel1.
2. Make TextBox1 bigger than Panel1 and fill it with a bunch of text.
3. Set Panel1.AutoScroll to true.
4. Add a Button1 to Form1 and set it's TabIndex to 0 to grab the focus on loading.

Run the project, move the Panel1 scrollbars around and then click on some text in TextBox1. TextBox1 will jump annoyingly as Panel1 tries to scroll the TextBox's upper left corner into view. Now place the code below into Form1 and repeat the test. Much nicer! This worked in VB 2010 Express.

Delegate Sub AutoScrollPositionDelegate(ByVal sender As ScrollableControl, ByVal p As Point)
Private Sub TextBox1_Enter(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox1.Enter

   Dim p As Point = Panel1.AutoScrollPosition
   Dim del As AutoScrollPositionDelegate = New AutoScrollPositionDelegate(AddressOf SetAutoScrollPosition)

   Panel1.BeginInvoke(del, {Panel1, p})

End Sub
Private Sub SetAutoScrollPosition(ByVal sender As ScrollableControl, ByVal p As Point)

   p.X = Math.Abs(p.X)
   p.Y = Math.Abs(p.Y)
   sender.AutoScrollPosition = p

End Sub