It suffers from the same bug as the stock bootstrap modal; focus is not contained in the popup window. Press tab a few times and soon your keyboard focus is on elements on the page behind the modal.
I didn't understand that decision back then and I don't understand it today. The patch weighs in at about 100 lines which doesn't seem heavy to me. Especially when you consider that without the patch the bootstrap-"modal" is not a modal...
Anyway, here's the fixed bootstrap-modal.js for the version that was current back then:
https://gist.github.com/0d04646ab6cf4df50610
Line 23-82 is the important bit, that snippet is lifted nearly 1:1 from jquery.ui.dialog which fixed this issue ages ago. Beyond that there's only a couple lines of glue code.
Feel free to port this forward to the current version (or better-modal). Personally I've stopped using the javascript parts of bootstrap because I don't want to maintain forks for basic bugs like that.
modal.attr('tabindex','1').style('outline','0'); modal.focus();
That: 1 - enables focus on the modal div 2 - hides focus visual indicator on the modal (because it's not an interactive element, just a wrapper) 3 - sets focus on the modal
The user's next tab keypress will select the first focusable element inside the modal.
Restricting tab keypress to just elements inside the modal (ie cycling through only modal content) doesn't return significant benefit and it's a whole lot more work. It's easy for a user to error-correct by simply clicking back inside the modal, but it's hard for a developer to manage enabled/disabled states for native behavior.