I run TWiki 5.0 in my Web site. Everything works in Red Hat Linux 5.0 and Windows XP. But under Windows 7, the edit button on a TWiki Web/Topic does not allow use of the left button on the mouse for locating the cursor in the edit page and subsequent highlighting of words for editing.. One must go into Raw Edit. In Raw Edit both mouse buttons work correctly. This is not an issue under Windows XP or Linux. I don't know if this issue exists for higher versions of TWiki under Windows 7. I want to stay with TWiki 5.0 because of concerns associated with upgrading.
The TinyMCE editor that currently ships with TWiki is a bit outdated. We need to upgrade it to the latest version. This is likely to fix the issue you report. The issue is likely the same in TWiki-4.1 onwards, e.g. installing TWiki-5.1 would not fix the issue.