Fix empty inspector popout bug in Firefox

Due to some sort of mysterious race condition (probably deferred reflow)
calling this.$menu.show() set style="" rather than style="display:block;"
because Firefox believed the parent was still hidden.

Work around this by removing display:none; from the context's CSS rules.
We don't need it because we always manually .hide() these things anyway.

Bug: 50073
Change-Id: I29e210aba9a6265d8364ff8ae49408cb4c2428b9