The size of RadForm under Windows XP is incorrect when its initial state as MDI child is Maximized and after that the state is changed to Normal.

Workaround:
Imports Telerik.WinControls.UI

Public Class BaseRadForm
    Inherits RadForm 

    Private Shared LastWindowState As FormWindowState = FormWindowState.Normal
    Private Shared suspendClientSizeChangedFlag As Boolean = False
 
    Protected Overrides Sub OnResizeBegin(ByVal e As System.EventArgs)
        Me.MaximumSize = New Size(0, 0)
        MyBase.OnResizeBegin(e) 
    End Sub
     
    Protected Overrides Sub OnActivated(ByVal e As System.EventArgs)
        Me.MaximumSize = New Size(0, 0)
        MyBase.OnActivated(e)
    End Sub

    Protected Overrides Sub OnDeactivate(ByVal e As System.EventArgs)
        Me.MaximumSize = New Size(0, 0)
        MyBase.OnDeactivate(e)
    End Sub

    Protected Overrides Sub OnClientSizeChanged(ByVal e As System.EventArgs)
        Dim osInfo As System.OperatingSystem = System.Environment.OSVersion
        If (suspendClientSizeChangedFlag OrElse osInfo.Version.Major >= 6) Then
            Return
        End If

        If Not LastWindowState.Equals(Me.WindowState) Then
            LastWindowState = Me.WindowState
            If Me.WindowState.Equals(FormWindowState.Normal) Then
                suspendClientSizeChangedFlag = True
                Me.MaximumSize = New Size(500, 500)
                suspendClientSizeChangedFlag = False
            ElseIf Me.WindowState.Equals(FormWindowState.Maximized) Then
                Me.MaximumSize = New Size(0, 0)
            End If
        Else
            Me.MaximumSize = New Size(0, 0)
        End If

        MyBase.OnClientSizeChanged(e)

    End Sub


End Class

When the RadForm is used as MDI Child hosted by RadDock, the rendering of the form is not appropriate.

To reproduce:
- Add four buttons to a RadForm
- On each button click use ThemeResolutionService to change the theme
=> the form size increases in height

Workaround:
Set the MaximumSize of the form to the current form size prior changing the theme and then remove this setting:
private void radButton1_Click(object sender, EventArgs e)
{
    this.MaximumSize = this.Size; ;
    ThemeResolutionService.ApplicationThemeName = "Office2010Blue";
    this.MaximumSize = Size.Empty;
}

 This issue starts happening as of Q2 (2025.2.520) version.

Workaround: override SetClientSizeCore() inside of inherited RadForm

public partial class RadForm2 : RadForm1
{
    protected override void SetClientSizeCore(int x, int y)
    {
        base.SetClientSizeCore(x, y);

        if ((!this.IsLoaded || !this.IsHandleCreated) &&
            this.IsInitialized)
        {
            MethodInfo mi = typeof(Form).GetMethod("SetClientSizeCore", BindingFlags.NonPublic | BindingFlags.Instance);
            if (mi != null)
            {
                IntPtr ptr = mi.MethodHandle.GetFunctionPointer();
                Action<int, int> baseSetClientSizeCore = (Action<int, int>)Activator.CreateInstance(
                    typeof(Action<int, int>), this, ptr);

                baseSetClientSizeCore.Invoke(x, y);
            }
        }
    }
}

How to reproduce: set the Size property of the form in the designer of Visual Studio, pay attention to the serialized ClientSize value. Run the form and check the ClientSize, it has increased

Workaround: 
public partial class Form2 : RadForm
{
    public Form2()
    {
        InitializeComponent();
        Padding p = TelerikDpiHelper.ScalePadding(this.FormBehavior.ClientMargin, new SizeF(1f / this.RootElement.DpiScaleFactor.Width, 1f / this.RootElement.DpiScaleFactor.Height));
        this.MaximumSize = new Size(this.Size.Width - p.Horizontal, this.Size.Height - p.Vertical- this.FormBehavior.ClientMargin.Bottom + this.FormElement.TitleBar.Size.Height);
    }

    protected override void OnShown(EventArgs e)
    {
        base.OnShown(e);

        this.MaximumSize = new Size(0, 0);
    }
}

To reproduce:
- Set the StartPosition to CenterScreen
- Show the form on a HDPI monitor

Workaround:
var form = new RadForm();

float dpiX, dpiY;
Graphics graphics = this.CreateGraphics();
dpiX = graphics.DpiX /100;
dpiY = graphics.DpiY /100;

form.StartPosition = FormStartPosition.Manual;
var monSize = Screen.FromControl(this).Bounds;
var centerX = (monSize.Width / 2) - (form.DesktopBounds.Width * dpiX / 2);
var centerY = (monSize.Height / 2) - (form.DesktopBounds.Height * dpiY/ 2);
form.Location = new Point((int)centerX,(int) centerY);
form.Show();

How to reproduce: just create a RadForm, in the designer change its size and set its FormBorderStyle property to be None. When the form loads its size will be increased.

Workaround: set its MaximumSize property
this.MaximumSize = new Size(400, 80);

Please refer to the attached screenshot and sample project.

Workaround: call the RadForm.EndInit method before showing the form.

Workaround: set the RadRibbonFormBehavior1.AllowTheming property of the form to false

public class RadForm1
{
	public RadForm1()
	{
		InitializeComponent();

		this.RadRibbonFormBehavior1.AllowTheming = false;

	}
}

In this particular case, we have custom RadButtonElements placed in the title bar of the form. We have subscribed to the click event on each button. In the click event handlers, we are showing file dialogs. When clicking one time to show a dialog and the mouse leaves the title bar bounds, this triggers the click event twice, thus opening another dialog.

As a workaround, we could use the MouseDown event instead or raise a flag in the click event handler.

To reproduce, use the following code and minimize the child form:

    public partial class RadForm1 : Telerik.WinControls.UI.RadForm
    {
        public RadForm1()
        {
            InitializeComponent();
            this.IsMdiContainer = true;
 
            RadForm form = new RadForm();
            form.Text = "MDI Child";
            form.MdiParent = this;
            form.MinimumSize = new Size(800,600);
            form.Show(); 
        }
    }

To reproduce:

1. Create a new .NET 7 project and add a RadForm to it with a single control (e.g. RadButton).

2. Open the designer and have a look at the form's Size.

3. Move the button and save the changes

4. Close the designer and open it again.

Expected result: the RadForm's Size is unchanged

Actual result: the RadForm gets smaller, its height is reduced.