Steps to Reproduce

Scenario 1

1. Ensure your IE window is maximized
2. Edit a page and go to Wiki Markup editor
3. normal_maximized_IE.png. The vertical scroll bar should be there.
4. Restore down the IE window.
5. broken_restored_down_IE.png. The vertical scroll bar disappeared.

Scenario 2

1. Ensure your IE window is restored down (not maximized)
2. Edit a page and go to Wiki Markup editor
3. normal_restored_down_IE.png. The vertical scroll bar should be there.
4. Maximize the IE window.
5. broken_maximized_IE.png. The vertical scroll bar is misplaced.

Findings

This problem seems to be only reproducible in Wiki Markup Editor and in IE 8.

Workaround

1. Turn on Compatibility View on IE (via Tools > Compatibility View). You can add your Confluence to be always displayed in Compatibility View at Tools > Compatibility View Settings.
2. Switch to Rich Text editor or Preview, then switch back to Wiki Markup editor.