While testing our main branch (https://twbs-bootstrap.netlify.app) on Firefox for Android, I noticed that when the docs menu is opening/closing, the transition is very sluggish. Chrome has better performance, but it's still a little sluggish. Now, the device I use is not a low end I believe, Samsung Galaxy S9, and I also tested out a Galaxy S20, both with Firefox and Chrome.
I believe this fixes the issue since it reduces layout reflows. As a bonus, the image will be cached. I used an empty alt attribute since I believe the image is decorative only, but @patrickhlauke let me know if something else is needed.
Note that we didn't see the issue before because we were using a horizontal navbar approach.
@mdo I added this in the alpha2 project because its effect is pretty big.
Preview: https://deploy-preview-31761--twbs-bootstrap.netlify.app/