Use a browser which displays scrollbars, such as Firefox on Windows. Using default Clean theme, set up a course that contains blocks on the right. Turn editing on. Click 'Add an activity or resource', then cancel the dialogue (and repeat as necessary) while observing the right-hand blocks EXPECTED: The blocks on the right should remain in exactly the same position BEFORE FIX: The blocks on the right shift slightly Find or create a course with many questions in the question bank. Add a quiz and go to the Edit quiz page. Click Add -> From question bank Verify that the main browser window has a scroll bar. (If not, the question bank is not showing enough questions. Try the Show all 123 link at the bottom, and/or the Search options -> Also show questions from subcategories setting.) EXPECTED: The blocks on the right should remain in exactly the same position BEFORE FIX: There was no shift in this case. (This is to make sure we have not introduced a regression.) Note: In Clean theme, the header bar is aligned with the browser viewport (not body) so that still shifts slightly. However this fixes the main part of the problem. To test more comprehensively, repeat on other browsers including those which don't count the space as wider without scrollbars (and didn't have the problem before or after this change). For example, IE11 in Windows appears to be in this category.