I had this issue as well (bootstrap 3.1.1). I was opening a modal and there was a missing space on the backdrop (where a scroll bar will appear if the modal is greater than page height) and the content of the page was resizing and shifting to the left.
My layout uses a fixed navbar.
I added a couple of CSS selectors that seems to prevent the page resizing and ensuring that the modal-backdrop fills the screen
html {
/* This prevents the page from shifting when a modal is opened e.g. search */
overflow-y: auto;
}
.modal,.modal.in,.modal-backdrop.in {
/* These are to prevent the blank space for the scroll bar being displayed unless the modal is > page height */
overflow-y: auto;
}
I still find it a bit odd where you can have two scroll bars if the page and the modal content is more than the screen height but I can live with that.