Use attached to reproduce.

Workaround:
RadTextBox tb = new RadTextBox();

tb.Font = new Font(Font.Name, 12F, FontStyle.Bold);

tb.MaxLength = 200;
tb.Name = "textBox1";
tb.Text = "<Required>";
tb.Location = new Point(49, 7);
var pixels = tb.Font.SizeInPoints * 96 / 72;
tb.TextBoxElement.TextBoxItem.HostedControl.MinimumSize = new Size(0, (int)pixels + 6);

The new functionality should prevent duplicates to be added as tokens in the editor. An already selected item should not be visible in the popup as well. 

The attached 1148813-AutocompleteDuplicates.zip project features a possible custom solution. 

Windows TextBox - OSK perfect

Telerik RadTextBox - OSK perfect

Telerik RadTextBoxControl - OSK doesn't work

Telerik RadDropDownList - OSK doesn't work

Telerik AutoCompleteBox - OSK doesn't work

This should work for all controls that have text input.

The suggestion must popup after every word.
There should be an option for positioning the drop down inside the textbox.

 

RadSpellChecker is also using MEF to load the default dictionary: https://docs.microsoft.com/en-us/dotnet/framework/mef/attributed-programming-model-overview-mef If MEF fails the dictionary will not load and the control will not spell check.

Workaround:
this.radSpinEditor1.CausesValidation = false;
((TextBox)this.radSpinEditor1.SpinElement.TextBoxItem.HostedControl).CausesValidation = false;

How to reproduce: add a spellchecker to the form and configure it to spellcheck a multi line text box with a very long text.

Workaround: use RadTextBoxControl

How to reproduce: run the attached project on a system with an increased scaling - 200%

Workaround: the null text needs to be set at run-time and after subscribe the control to the CreateTextBlock event

public class MyTextBlock : TextBlockElement
{
    public override void DpiScaleChanged(SizeF scaleFactor)
    {
        base.DpiScaleChanged(scaleFactor);
        this.ResetLayoutCore();
    }
} 

this.radTextBoxControl1.TextBoxElement.CreateTextBlock += this.RadTextBoxControl1_CreateTextBlock; 
private void RadTextBoxControl1_CreateTextBlock(object sender, CreateTextBlockEventArgs e)
{
    e.TextBlock = new MyTextBlock();
} 

How to reproduce: check the attached project and video

Workaround: set a minimum size to the control
 public partial class RadForm1 : RadForm
 {
     public RadForm1()
     {
         InitializeComponent();

         this.radTextBox1.MinimumSize = new Size(0, 20);
     }
 }

Using UI for WinForms R3 2017 (version 2017.3.912)

Strange behavior while selecting text with the mouse, while TextAlign is set to Right and there is more text then visible.
To reproduce try selecting all the text in the control from the attached file (once to the left, once to the right).

If the ShowClear button is visible and RihtToLeft is true, it won't select to the left (while there is more text than visible).


RadTextBoxControl radTextBoxControl1 = new Telerik.WinControls.UI.RadTextBoxControl();

this.radTextBoxControl1.Location = new System.Drawing.Point(239, 186);
this.radTextBoxControl1.Name = "radTextBoxControl1";
this.radTextBoxControl1.RightToLeft = System.Windows.Forms.RightToLeft.Yes;
this.radTextBoxControl1.ShowClearButton = true;
this.radTextBoxControl1.Size = new System.Drawing.Size(64, 20);
this.radTextBoxControl1.TabIndex = 6;
this.radTextBoxControl1.Text = "123456789123456789";
this.radTextBoxControl1.TextAlign = System.Windows.Forms.HorizontalAlignment.Right;

To reproduce: please refer to the attached sample screenshots and project.

Note: this issue is relevant for RadDropDownList as well.

Workaround:             
            this.radDropDownList2.DropDownListElement.TextBox.TextBoxItem.Multiline = true;
            this.radDropDownList2.DropDownListElement.TextBox.TextBoxItem.MinSize  = new Size(30, 15); 

How to reproduce: also check the attached video
this.radMaskedEditBox1.MaskType = MaskType.Numeric;

Workaround:
private void RadMaskedEditBox1_TextChanged(object sender, EventArgs e)
{
    if (this.radMaskedEditBox1.Text == "-0")
    {
        if (this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.Text.Length == 2)
        {
            //Workaround
            this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.Text = this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.Text.Remove(1, 1);
        }
    }
}

How to reproduce: 
radDateTimePicker1.DateTimePickerElement.ShowTimePicker = true;
radDateTimePicker1.Format = DateTimePickerFormat.Custom;
radDateTimePicker1.DateTimePickerElement.CalendarSize = new Size(330, 250);
radDateTimePicker1.ReadOnly = true;

Workaround: 
this.radDateTimePicker1.DateTimePickerElement.ShowTimePicker = true;
this.radDateTimePicker1.Format = DateTimePickerFormat.Custom;
this.radDateTimePicker1.DateTimePickerElement.CalendarSize = new Size(330, 250);
this.radDateTimePicker1.ReadOnly = true;

RadDateTimePickerCalendar calendarBehavior = this.radDateTimePicker1.DateTimePickerElement.GetCurrentBehavior() as RadDateTimePickerCalendar;
calendarBehavior.TimePicker.TimePickerElement.ReadOnly = true;

RadDataLayout.LayoutControl has Text property and it is not relevant to show RadMarkupEditor when changing the text.

The workaround in the following MSDN resources results in an incorrect behavior in the header of RadDateTimePicker: https://code.msdn.microsoft.com/Fixing-Persian-Locale-for-6e66e044#content

To reproduce: run the attached sample project, open the popup and try to to scroll the grid with using the mouse wheel. You will notice that the popup is immediately closed.

Workaround: 
    Private Sub PopupClosing(sender As Object, args As Telerik.WinControls.UI.RadPopupClosingEventArgs)
        If args.CloseReason = Telerik.WinControls.UI.RadPopupCloseReason.Mouse Then
            args.Cancel = True
        End If
    End Sub