When the TitleBarVisibility is Collapsed, the backstage close button is aligned with the top of the backstage adorner. The button should have a top margin applied. 
This reproduces when the BackstagePosition property of RadRibbonBackstage is set to Office2013, which is the default setting of newer Telerik themes.

To work this around, get the backstage close button and set its top margin.

 private void RadRibbonBackstage_Loaded(object sender, RoutedEventArgs e)
 {
     var backstage = (RadRibbonBackstage)sender;
     var closeButton = backstage.FindChildByType<RadRibbonButton>();
     closeButton.Margin = new Thickness(closeButton.Margin.Left, 15, closeButton.Margin.Right, closeButton.Margin.Bottom);
 }

The width of the popup element that shows the selected tab content when RadRibbonView is minimized is wrong in the following situation. A tab in the minimized ribbon is clicked to display its content, then the tab is clicked again. After that, the width of the ribbon is changed. At this point when you open the minimized content, the popup uses the previous size of the RadRibbonView control. 

To work this around, you can manually update the MaxWidth of the popup content element on size changed.

public class CustomRibbonView : RadRibbonView
{
    private ContentPresenter popupContent;

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        this.popupContent = this.GetTemplateChild("SelectedTabContentPopup") as ContentPresenter;
    }

    protected override void OnRenderSizeChanged(SizeChangedInfo sizeInfo)
    {
        base.OnRenderSizeChanged(sizeInfo);
        if (popupContent != null)
        {
            double popupContentPadding = 16;
            popupContent.MaxWidth = this.ActualWidth - popupContentPadding;
        }
    }
}

Currently, the RadRibbonGallery control provides the PopupViewportHeight property that sets the Height property of the ScrollViewer, which hosts the RadGalleryItem instances when it is expanded.

We could introduce a property, for example, PopupViewportMaxHeight that will set the MaxHeight property of the ScrollViewer. This way, if the height gets higher than the value specified for this property, a vertical scrollbar should appear.

Currently, the RadRibbonTab GetChildrenCore method doesn't create automation peers for the element in the ContentPresenter for the TabStripAdditionalContent.

At this point, you can enable this by creating a custom AutomationPeer that derives from RadGridViewAutomationPeer and override its GetChildrenCore() method. This way you can manually create the peer for the AdditionalTabStripContent. This idea is shown in the attached project.

By default the drop down buttons (like RadRibbonDropDownButton and RadRibbonSplitButton are opening their drop down content on mouse enter, when hosted in ApplicationMenu. When a drop down of a button is opened and you hover another button, the currently opened drop down should get closed. But this doesn't happen when the ApplicationMenu is populated through its ItemsSource and the buttons are defined in the ItemTemplate.

To work this around avoid using ItemsSource and ItemTemplate. Instead add the buttons directly in the Items collection of ApplicationMenu.

The RibbonSplitButton can behave like a toggle button by setting its IsToggle property to True. When the button is hosted in ApplicationMenu its default template is changed with a special one that adjust the UI of the button to the ApplicationMenu design. However, the alternative template is missing the code that updates the UI of the button when the IsChecked property is True.

To work this around, you can edit the ControlTemplate of RadRibbonSplitButton. The template for the button when hosted in ApplicationMenu is using the following x:Key "ApplicationRibbonSplitButtonTemplate". You can apply the edited template through the Template property of RadRibbonSplitButton.

The RadRibbonWindow titlebar is higher than expected when the Windows 11 theme is used. 

A side effect of this is that the content overlaps the titlebar, when the content is not a RadRibbonView. 

This reproduces when the Windows11 and Crystal themes are used. To reproduce it, the IsWindowsThemeEnabled property should be set to false in order for the Telerik theme to get applied to the window.

To work this around, you can use the visual tree helper methods in the Loaded event of RadRibbonWindow in order to get the corresponding visuals and change their size and offset.

private void RadRibbonWindow_Loaded(object sender, RoutedEventArgs e)
{
	var windowDecoratorPanel = this.ChildrenOfType<Grid>().FirstOrDefault(x => x.Name == "MaximizeWindowDecorator");
	var titleBarPanel = windowDecoratorPanel.FindChildByType<Grid>();
	titleBarPanel.RowDefinitions[0].Height = new GridLength(30);

	var clientArea = windowDecoratorPanel.ChildrenOfType<Border>().FirstOrDefault(x => x.Name == "PART_ClientAreaBorder");
	clientArea.Margin = new Thickness(0);
}

The ScreenTip no longer is displayed on mouse over of the owner element. This reproduces only if the element is hovered before the ScreenTips of the previously hovered element is closed. In other words this happens if the two elements that contain ScreenTips are overlapping are very close to one another and there is no space that allows closing the tooltip while moving the mouse.

To work this around, subscribe to the Opened event of the ScreenTip control and set its Visibility to Visible if the element was previously Collapsed.

<Window.Resources>
	<Style TargetType="telerik:ScreenTip">
		<EventSetter Event="Opened" Handler="ScreenTip_Opened" />
	</Style>
</Window.Resources>

private void ScreenTip_Opened(object sender, RoutedEventArgs e)
{
	var screenTip = (ScreenTip)sender;
	if (screenTip.Visibility == Visibility.Collapsed)
	{
		screenTip.Visibility = Visibility.Visible;
	}
}

Setting the LayoutMode to Simplified doesn't do anything in the RadRibbonTabs that are assigned to a RadRibbonContextualGroup.

To work this around, extract the "ContextualTabsTemplate" ControlTemplate and the following elements in the "RibbonScrollViewer".

<telerikRibbonViewPrimitives:RibbonScrollViewer x:Name="TabItemsScrollViewer">
	<Grid>
		<ItemsPresenter x:Name="PART_DefaultItemsPresenter" HorizontalAlignment="Left" />
		<ItemsControl x:Name="PART_SimplifiedItemsControl" Visibility="Collapsed" 
				  ItemsSource="{Binding SimplifiedItems, RelativeSource={RelativeSource TemplatedParent}}">
			<ItemsControl.ItemsPanel>
				<ItemsPanelTemplate>
					<telerikRibbonViewPrimitives:RibbonGroupsPanel />
				</ItemsPanelTemplate>
			</ItemsControl.ItemsPanel>
		</ItemsControl>
	</Grid>
</telerikRibbonViewPrimitives:RibbonScrollViewer>

Then define the following DataTrigger in the ControlTemplate.Triggers collection:

<DataTrigger Binding="{Binding RelativeSource={RelativeSource AncestorType=telerikRibbonView:RadRibbonView}, Path=LayoutMode}" Value="Simplified">
	<Setter TargetName="PART_SimplifiedItemsControl" Property="Visibility" Value="Visible"/>
	<Setter TargetName="PART_DefaultItemsPresenter" Property="Visibility" Value="Collapsed"/>
</DataTrigger>

Then, assign the custom ControlTemplate via the Template property of RadRibbonTab.

<telerik:RadRibbonTab Template="{StaticResource CustomContextualRibbonTabTemplate}" />

Allow showing the Quick Access Toolbar (QAT) items in the customization drop down menu (the quick access menu). This is the menu that shows the "Minimize the Ribbon" and the "Show below the Ribbon" options. 

The new feature should allow to display all items from the QAT in the drop down. Clicking an item from the drop down should show or hide it in the QAT.

The attached project shows one way to get this behavior with custom code.