User is unable to navigate to 'upload' button in 'insert image' dialogue. On invoking 'insert image' dialogue focus is navigating in the background and after navigating whole page then focus is going to the 'insert image' dialogue. Now, when user try to upload the image, focus is going out of the dialogue (going to the top of the page).
When focus order does not follow the visible reading order, the experience can create a different interaction pattern, and/or unintended content relationships for assistive technology users. On rare occasions, there is a logical reason the focus order is different from the reading order, but the meaning of the content and interactions must remain the same.