6754b0e5c8
As the comment in the method, `CurrentBlockBoundary` may be set when `mContent` is not a block. However, it's not allowed that there is an editable block ancestor of `mContent` in same editing host (in this case, `mContent` should be the block). Therefore, it should compute ancestor block element and check whether it's not in same editing host if `mContent` is editable and connected. Differential Revision: https://phabricator.services.mozilla.com/D190644