C# Panel with autoscroll – Srollbar position reset on a control focus

cwinforms

This is for a windows form.

Panel has AutoScroll = True

I am adding panels dynamically to the main panel which end up exceeding the main panel display rectangle.
Then adding Labels, Combo Boxes, and Text Boxes to the added panels.

If I select a Combo Box or Text Box it resets the main panels scroll bar position to 0 and also the drop down menu for a Combo Box will be placed on the screen X,Y where it should be if the scrollbar had not reset.

I am thinking of saving the scroll position when a control is selected. After testing it seems the scroll position is not reset yet so I am able to capture it here. Then restoring the scroll position on some event of the panel I hope. I am trying to find out exactly what event I would use to restore the scroll position. I also hope that the drop down menu will be placed at the correct x,y when I do this.

A better solution would be to create a custom control based off the panel control and override an event possibly? This way I would not need to clutter up my project with saving the scroll position every time I use a scrolling panel.

Best Answer

I found the answer to the problem I was having here: Answer

public class CustomPanel : System.Windows.Forms.Panel
{
    protected override System.Drawing.Point ScrollToControl(System.Windows.Forms.Control activeControl)
    {
        // Returning the current location prevents the panel from
        // scrolling to the active control when the panel loses and regains focus
        return this.DisplayRectangle.Location;
    }
}

Related Solutions

C# – Scrolling problem with a WebBrowser control contained in a Panel control

I had the same problem, also with a WebBrowser inside a Panel. Here's the solution I'm using (which I found somewhere else on stackoverflow):

class AutoScrollPanel : Panel
{
    public AutoScrollPanel()
    {
        Enter += PanelNoScrollOnFocus_Enter;
        Leave += PanelNoScrollOnFocus_Leave;
    }

    private System.Drawing.Point scrollLocation;

    void PanelNoScrollOnFocus_Enter(object sender, System.EventArgs e)
    {
        // Set the scroll location back when the control regains focus.
        HorizontalScroll.Value = scrollLocation.X;
        VerticalScroll.Value = scrollLocation.Y;
    }

    void PanelNoScrollOnFocus_Leave(object sender, System.EventArgs e)
    {
        // Remember the scroll location when the control loses focus.
        scrollLocation.X = HorizontalScroll.Value;
        scrollLocation.Y = VerticalScroll.Value;
    }

    protected override System.Drawing.Point ScrollToControl(Control activeControl)
    {
        // When the user clicks on the webbrowser, .NET tries to scroll to 
        //  the control. Since it's the only control in the panel it will 
        //  scroll up. This little hack prevents that.
        return DisplayRectangle.Location;
    }
}

C# – Avoid Flickering in Windows Forms

Yet another solution:

//TODO: Don't forget to include using System.Runtime.InteropServices.

internal static class NativeWinAPI
{
    internal static readonly int GWL_EXSTYLE = -20;
    internal static readonly int WS_EX_COMPOSITED = 0x02000000;

    [DllImport("user32")]
    internal static extern int GetWindowLong(IntPtr hWnd, int nIndex);

    [DllImport("user32")]
    internal static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);
}

And your form constructor should look as follows:

public MyForm()
{
    InitializeComponent();

    int style = NativeWinAPI.GetWindowLong(this.Handle, NativeWinAPI.GWL_EXSTYLE);
    style |= NativeWinAPI.WS_EX_COMPOSITED;
    NativeWinAPI.SetWindowLong(this.Handle, NativeWinAPI.GWL_EXSTYLE, style);
}

In the code above, you might change this.Handle to something like MyFlickeringPanel.Handle

You can read a bit more about it here: Extended Window Styles and here: CreateWindowEx.

With WS_EX_COMPOSITED set, all descendants of a window get bottom-to-top painting order using double-buffering. Bottom-to-top painting order allows a descendent window to have translucency (alpha) and transparency (color-key) effects, but only if the descendent window also has the WS_EX_TRANSPARENT bit set. Double-buffering allows the window and its descendents to be painted without flicker.

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