This is a bug in MATLAB 8.0 (R2012b) and later releases. This issue may appear in MATLAB R2012a as well. This issue may be related to the new Help browser.
Beginning in MATLAB R2012b, the Help Browser uses a different renderer called the JxBrowser, which provides an enhanced help browsing experience. Because this may be an issue with the JxBrowser, try disabling the JxBrowser and instead use the HTML renderer (which may provide a slightly degraded Help Browser experience).
Launch a new session of MATLAB, and try to disable JxBrowser completely during this session, which can be done by executing the following command at the beginning of a MATLAB session:
This will set a preference disabling JxBrowser, which can be undone with the command:
NOTE: This bug has been fixed in MATLAB R2015a and later versions. Here is a link to its external bug report: