Top navigation bar in JIRA disappear.
When using JIRA 5.x, 6.0.x in the View Issue screen, some users complain about the top navigation bar disappearing.
The top navigation bar disappears and you are not able to perform new operations.
In JIRA 5.x, 6.0.x this behaviour occurs when the user scrolls down while the View Issue page is still loading.
Wait for the entire page to load or upgrade to JIRA 6.1 where this problem does not occur anymore since the structure of the View Issue page was modified.