Odd glitch with opacity / bootstrap modal
I was seeing the same issue after Chrome updated to 61. To workaround this I temporarily swapped the opacity on modal backdrops for rgba(0,0,0,0.5) background. Note that rgba background is unsupported in IE8 so you'll either want to add a fallback or not utilize the workaround in that environment.
I added style and it helped to avoid this bug
.modal-backdrop { opacity: 1 !important; background-color: rgba(0,0,0,0.5) !important;}
i'm seeing the same issue (black squares/artefacts) as well (since Chrome 61). I figure it's a rendering issue in Chrome.As it is annoying to our customers we are looking for a workaround but other than removing all opacity styles we have no solution yet.