* On some systems (currently hypothesized to be Firefox with
GTK+ using a system theme that has always-visible scrollbars with
a minimum size on their long axis), the suggestion menu starts off
visible and obscuring the chat message entry area, then moves to
obscuring the chat history area once it has been invoked. At no
time does it become invisible.
* Fix, by hiding the suggestion menu by default and explicitly
managing its visibility as it is updated.
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