the following exceptions occur when upgrading to Xamarin.Forms v 4.0:

System.NotSupportedException: Unable to activate instance of type Telerik.XamarinForms.DataControlsRenderer.Android.ListView.TemplateCellContainer

System.NullReferenceException: Object reference not set to an instance of an object
  at Telerik.XamarinForms.InputRenderer.iOS.SegmentedControlRenderer.OnElementDetached

when placement target is a StackLayout/GridLayout the popup is not visualized correctly.

For example:

<ContentPage.Content>
    <StackLayout>
        <Entry Text="{Binding Path=Username, Mode=TwoWay}"/>
        <telerikPrimitives:RadPopup.Popup>
            <telerikPrimitives:RadPopup x:Name="popup"
                                    IsModal="True"
                                    OutsideBackgroundColor="#6F000000" 
                                    IsOpen="{Binding Path=PopUpShow, Mode=TwoWay}">
                <telerikPrimitives:RadBorder CornerRadius="8" 
                                            BackgroundColor="Wheat">
                    <Grid Padding="20">
                        <Grid.RowDefinitions>
                            <RowDefinition Height="30" />
                            <RowDefinition Height="20" />
                            <RowDefinition Height="40" />
                        </Grid.RowDefinitions>
                        <Button Grid.Row="0"
                            Padding="2"
                            HorizontalOptions="End" 
                            Text="Send"
                            Clicked="ClosePopup"/>
                        <Label Grid.Row="2" Text="{Binding Path=PopUpMessage}" />
                    </Grid>
                </telerikPrimitives:RadBorder>
            </telerikPrimitives:RadPopup>
        </telerikPrimitives:RadPopup.Popup>
    </StackLayout>
</ContentPage.Content>

Workaround: set VerticalOptions="StartAndExpand" to the StackLayout

SlideView item does not update after navigation between pages

After scrolling down enough times where all of the data is loaded, if I try to scroll a bit more, things go a bit crazy.  A loading icon flickers on and off over the grid; the data in the grid flickers away and then back; and the entire UI essentially gets stuck in this situation.

On UWP and Android the item can be swiped beyond the SwipeOffset property. Add option to restrict the swiped distance to the SwipeOffset value.

Consider implementing a Completed event that is raised when the Completed event of the entry is raised. This is in order to accommodate customer needs to get notification for enter pressed.

a new component - Image viewer.
this component will take as source images meta data and show images gallery, with view / templates.
(carousel / grid for instance), 
allowing tap to zoom full screen and drag to exit zoom

System.ArgumentNullException(Value cannot be null) is thrown when item from the listview is selected and the groups are expanded/collapsed.

When you have some kind of image - CachedImage, FFimage, XF Image, within the ItemTemplate of the RadListView and you want to use the ItemSwipe functionality - the items constantly flicker. 

If you set a constant height to the listView item - the issue is not observed:

QSF DataGrid CRUD example
When editing the value inside the OrderDate column, the edited value is not presented in the OrderDate column. It keeps the old value

Provide a way to set header and footer that are always visible in the ListView and are excluded from the scrolling.

Flicker can be observed when datagrid is manipulated/updated, or entering in edit mode. One example is tapping on a column header - when there is a column with a HeadetTemplate, the column header flickers. Other example is to double-tap a cell to go in edit mode. Also the behavior can be reproduced when DataGrid TemplateColumn is used.

Provide a tool that will upgrade the solution to a newer version of Telerik UI for Xamarin (similar as the WPF Project Upgrade Wizard)

Add a new image cropper control, which allows users to drag/zoom a selected image within a defined frame, so that the resulting image has a defined width and height.   This should offer an experience similar to the Facebook app's screen for adding/updating your profile photo.   The control should display the non-selected area of the photo with configurable opacity settings, and optionally allow for a resulting circular image.

When RadListView is placed inside a TabViewItem content of the RadTabView control,  it does not preserve its SelectedItem, scroll position and expanded items (if it is grouped).

Add support fir VisualElement.AccessKey like in the Xamarin.Forms controls:

https://docs.microsoft.com/en-us/dotnet/api/xamarin.forms.platformconfiguration.windowsspecific.visualelement?view=xamarin-forms

DataGridColumn.ActualWidth always returns 0

In DayView mode, when TimelineInterval is set to "0:30", in Android it shows just the hour component. Like 1 PM, 1 PM, 2PM, 2 PM etc.