While I think I understand why you added this new feature to more easily add a new card (I've seen many new users struggle with how to add a card), I'm afraid your implementation introduced a different usability issue.
The problem is that this new bar covers or obscures content in the browser because it appears over the top of the last line in the browser (I primarily use Chrome on a Mac). I've run into this in a couple different scenarios:
1. Using the browser's Find feature when the page is longer than the scrollable region or viewport. When the Find finds an occurance of the term not in the viewport, it auto-scrolls to that term such that it is displayed in the last line of the viewport... which puts it directly behind the Mingle Add Card Footer bar. At first I thought the browser find was broken...
2. When editing a page in Mingle and I entering content at the bottom (e.g. entering results of acceptance tests at the bottom of a card), again the last line of the viewport is covered. The workaround is to click outside the textedit box and scroll the browser window's viewport down, and then click back into the textedit box to resume typing (but this is pretty lame when you're a keyboard user).
Not sure what your options are for fixing this, but seems like it should be possible.
Oh, I also tried to just turn this new Add card bar off (the usability issues outweigh its benefits to me), but could not find a way to do that.
Thanks for listening / fixing this,