This allows a linked SplitPanels to be dynamically expanded and collapsed by clicking on the splitter button, and resized by dragging the splitter.

Use attached to reproduce. Move the splitter with the mouse and you will notice that is position is not correct when the mouse is released

The Modifiers property should be visible in the properties window when a SplitPanel instance is selected. 

Workaround:

Manually change the modifier in the Designer.cs file to protected.
  

Workaround: check the attached project

When you cut-copy RadSplitContainer containing two SplitPanels from one form to another, only the RadSplitContainer is pasted, but the SplitPanels are missing.

Add functionality to collapse and expand the split panels in RadSplitContainer by buttons positioned in the splitter area just like ASP.NET. See example here: 
http://demos.telerik.com/aspnet-ajax/splitter/examples/collapseexpand/defaultcs.aspx

SplitPanel - there is no property Name in Visual Studio designer.

The end user should be able to specify what Cursor to be used while resizing a panel.

Currently this can be achieved using the custom control below:
public class MyRadSplitContainer : RadSplitContainer
{
    private bool isResizing;
 
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadSplitContainer).FullName;
        }
    }
 
    protected override void OnMouseDown(MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left && this.ContentRectangle.Contains(e.Location))
        {
            SplitterElement splitter = GetSplitterElementAtPoint(e.Location);
            if (splitter != null && !splitter.Fixed)
            {
                this.isResizing = true;
            }
 
            base.OnMouseDown(e);
        }
    }
 
    protected override void OnMouseUp(MouseEventArgs e)
    {
        base.OnMouseUp(e);
 
        this.isResizing = false;
    }
 
    public override Cursor Cursor
    {
        get
        {
            Cursor cursor = base.Cursor;
            if (this.isResizing && cursor == Cursors.SizeWE)
            {
                cursor = Cursors.VSplit;
            }
            else if (this.isResizing && cursor == Cursors.SizeNS)
            {
                cursor = Cursors.HSplit;
            }
 
            return cursor;
        }
        set
        {
            if (value == Cursors.VSplit)
            {
                value = Cursors.SizeWE;
            }
            else if (value == Cursors.HSplit)
            {
                value = Cursors.SizeNS;
            }
 
            base.Cursor = value;
        }
    }
}

Please refer to the attached sample project. Double click the middle splitter in the horizontal split container.

Workaround: manually collapse the desired split panel:

private void Form1_Load(object sender, EventArgs e)
{
    this.radSplitContainer3.PanelCollapsing += radSplitContainer3_PanelCollapsing;
    foreach (SplitterElement s in this.radSplitContainer3.Splitters)
    {
        s.DoubleClick += s_DoubleClick;
    }
}

private void s_DoubleClick(object sender, EventArgs e)
{
    SplitterElement s = sender as SplitterElement;
    ((SplitPanel)s.RightNode).Collapsed = true;
}

How to reproduce:  check the attached video

Workaround: Only split panels can be added to RadSplitContainer,  so add a panel first to the control before using the document outline

Unable to cast object of type 'System.EventArgs' to type 'System.Windows.Forms.MouseEventArgs'.

Workaround: use the SpliPanel.Collapsed property to control whether the split panel is expanded or collapsed.

In design time when trying to copy-paste a RadSplitContainer, which contains a couple panels, it throws exception.

Steps to reproduce:
Add a RadSplitContainer to a form or user control.
Add a couple of panels to the container using the designer.
Press Ctrl+A to select all the controls on the form or user control.
Press Ctrl+C to copy them to the clipboard.
Create a new form or user control (this step isn't strictly necessary, but helps explain the issue).
Paste the controls on the clipboard into the new form or user control using Ctrl+V

To reproduce:
- Set the splitter size to 40.
- Drag the splitter without releasing the mouse.
- The mouse pointer is in the beginning of the splitter, but it should be in its center.

Sometimes there are 3 or more dots on a splitter, but when changing the orientation then there is only one dot.
sometimes there are lines, but when switching to disabled these lines are much bigger. But only in one orientation.

When you drop a RadSplitContainer in a RadDock ToolWindow, RadSplitContainer loses important options from its SmartTag's ActionMenu.
Similar behavior is obtained when you drag a RadSplitContainer from the ToolBox and drop it onto an empty RadDock.

Let's have a RadSplitContainer on a form. This RadSplitContainer should contain two SplitPanels and one splitter. Change the color of the splitter using the Edit UI Elements dialog. Run the project and you will get IndexOutOfRangeException. However, if you cut and paste the designer code in the form_load, it will work correctly.

Applying Settings to the RadSplitContainer >> SplitContainerElement >> SplitterFill element affect only the first Splitter instance in the RadSplitContainer.

Dear Sir/Madam

Unfortunately, the SplitContainer in the .NET version 5 has problems, as you can see in the picture, I can not change the size in design time.

Is there a newer version that fixes this problem?

Telerik Winform Version :2020.3.1020

Visual Studio Version : 16.8.3

Best Regards