Rationale:

While the popover create issue dialog may need to restrict the width of the entry boxes (discussion of this is being tracked: https://jira.atlassian.com/browse/JRA-28196), there is no need to do this in the full page issue creation screen.

Previous behaviour:

JIRA 5.2.11:

Right click on Create Issue, and open in new tab. The description field fill use the full width of the page.

JIRA 6.x:

Right click on Create Issue, and open in new tab. The description field use a fixed width, ignoring the width of the browser.