1. Go to https://www.telerik.com/kendo-angular-ui/components/editor#angular-editor-example
2. In the Angular Editor Example click on the first line of the first paragraph (let's say on the word Kendo)
3. Navigate to toolbar via Shift+Tab key
4. Use the arrow key to navigate to Format dropdownlist 
5. Use Enter to open the dropdownlist with the available formats (Paragraph, Heading 1, Heading 2, etc)
6. Choose e.g. "Heading 1" with Enter
7. Press Tab to focus the editor content (You can type some letters to ensure that the editor content is focused)
8. Press Shift+Tab to go back to toolbar
9. Press an arrow key (Left or Right) to try to navigate through the toolbar => Bug

Expected behaviour: It's possible to navigate through the toolbar items like in step 4
Experienced behaviour: You can not navigate through the toolbar items with arrow left or arrow right.

Important: After choosing an item from the dropdown, the kendo-dropdownlist has still an tabindex="0", what is also wrong. 

I observed this bug in our application but it's possible to reproduce it with your example, too.

Kind regards,

Roman