A user reported a significant performance impact with the RichTextBox in our recent update. The UI hangs repeatedly while typing.

Profiling showed a lot of garbage collections, and profiling highlighted the the Telerik class Telerik.Windows.Controls.MarkupExtensions.DocumentsResourceProvider.

#if NETCORE
        private static Style ThemeStyle
        {
            get
            {
                if (themeStyle == null)
                {
                    themeStyle = GetStyleFromApplicationTheme();
                }
                return themeStyle;
            }
        }

        private static Style GetStyleFromApplicationTheme()
        {
            if (StyleManager.IsEnabled && StyleManager.ApplicationTheme != null)
            {
                var themeName = StyleManager.ApplicationTheme.GetType().Name.Trim('_').ToLower();
                themeName = themeName.Replace("theme", string.Empty);

                var themesGeneric = Application.LoadComponent(new Uri("/Telerik.Windows.Controls.RichTextBox;component/themes/generic.xaml", UriKind.Relative)) as ResourceDictionary;
                if (themesGeneric != null)
                {
                    var currentThemeDictionary = themesGeneric.MergedDictionaries.FirstOrDefault(rd => rd.Source != null && rd.Source.OriginalString.ToLower().Contains(themeName));
                    if (currentThemeDictionary != null)
                    {
                        return currentThemeDictionary[typeof(DocumentsResourceProvider)] as Style;
                    }
                }
            }
            return null;
        }

I copied this code into my project to debug it, and found:

1. themeName is correctly calculated to "windows11".
2. themesGeneric is correctly loaded from the XAML.
3. currentThemeDictionary gets null because windows11 isn't included in the merged dictionaries.

As the method returns null, it means themeStyle is always null, and it will try again on the next caller. This code is called repeatedly while typing. Using my code (and reflection) to force this ThemeStyle to Fluent fixes the performance, though obviously is a huge workaround.

The scenario:

We have a document with text inside and we are executing the following steps:

1. Mouse left button down.
2. Select part of the text.
3. Replace the selection with new text programmatically (the text should be with a different length).
4. Release the left button.

Observed: Before releasing the mouse button the cursor position is correct but after releasing it the cursor position is changed to the initial selection end.

Copying content controls using DocumentFragment and then inserting them causes several undesired behaviors.

For example, if the original content control is in "Placeholder mode" (meaning the initial pre-generated text is intact), after the copy operation, its content is treated as user inserted and it is not fully selected on entering the control. Another issue is with Plain Text/Date Picket/Repeating Section control - after the copy, their Properties dialog cannot be shown.