The caret (text cursor) is entirely controlled and rendered by the browser. The RadEditor control does not implement any logic to affect its appearance. Any range and selection changes are done via native browser exposed API and the cursor is affected implicitly.
This might be a result of some browser add-on or non-updated browser version. If it happens only to one user, I suggest prompting for more details on the case. For example, browser version, trying to remove any add-ons etc.
Also, if the RadEditor controls are customized somehow with some JS logic, there might be a performance issue that could cause the browser to be unable to dispose the rendered caret.
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