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It appears that only the :Shift modifier doesn't work in Poltergeist.
Environment
Poltergeist gem 1.9
Phantomjs 2.1.1
OSX El Cap
Rails 4.2.6
The test code that appears to offend is:
#page.find('#category-text').send_keys [:Shift, '.']
page.find('#category-text').send_keys [:Alt, 'a']
In my code, I allow both shift-. (that is, the > symbol), and Alt-a to perform the same function. Both keys work in the browser (Chrome 50+, OSX), but only the 2nd line above works in the test.
Elsewhere in my code and tests, :Ctrl works fine.
Is this something peculiar to how :Shift works on an MBP perhaps?

The issue here is not the :shift key specifically, it's the use of '.' in combination with any modifier (:shift, :alt. :ctrl, etc...). The issue occurs because PhantomJS doesn't provide entries in its keyCode map for punctuation keys (https://github.com/ariya/phantomjs/commit/cab2635e66d74b7e665c44400b8b20a8f225153a) so they end up generating a keyCode of 0.
As a workaround you can call
page.find('#category-text').send_keys [:shift, :period]
which should provide the expected results.
Update: A fix for this is now in Poltergeist master and will be in the 1.11.0 release when it happens