Which WYSIWYG editor is used by xWiki? And was it ever using a different one? In which version did it change?
There are two WYSIWYG editors in the current (3.0-milestone-2) release: an older (now deprecated) one based on TinyMCE, and a GWT-based editor developed in-house, introduced in 1.7 (see http://platform.xwiki.org/xwiki/bin/Features/PageEditing#HWYSIWYGeditingmode and http://platform.xwiki.org/xwiki/bin/Features/WysiwygEditor for more details). Back in the very early days of XWiki (2005), there was another editor based on HTMLArea, but it was quickly replaced by TinyMCE.
So, the current editor is not an external one, but a custom developed one, available under the LGPL 2.1 license.
There are two WYSIWYG editors in the current (3.0-milestone-2) release: an older (now deprecated) one based on TinyMCE, and a GWT-based editor developed in-house, introduced in 1.7 (see http://platform.xwiki.org/xwiki/bin/Features/PageEditing#HWYSIWYGeditingmode and http://platform.xwiki.org/xwiki/bin/Features/WysiwygEditor for more details). Back in the very early days of XWiki (2005), there was another editor based on HTMLArea, but it was quickly replaced by TinyMCE.
So, the current editor is not an external one, but a custom developed one, available under the LGPL 2.1 license.