Resolve "Calendar: clicking top-right corner of an event hides it, even though there is no X there"

Since commit c4f2bb5f, close buttons are hidden by making them fully transparent rather than setting their visibility to false to keep the overall message layout stable. As a result, the buttons now work even when invisible, which is clearly unexpected - fix this by updating the reactive property appropriately.

Fixes #66 (closed)

Closes #66 (closed)