Support for overflow: hidden on the <body> element is quite limited in iOS and Android. To that end, when you scroll past the top or bottom of a modal in either of those devices' browsers, the <body> content will begin to scroll.
Also, note that if you're using inputs in your modal – iOS has a rendering bug which doesn't update the position of fixed elements when the virtual keyboard is triggered. There are a few work arounds for this including transforming your elements to position absolute or invoking a timer on focus to try to correct the positioning manually. This is not handled by Bootstrap, so it is up to you to decide which solution is best for your application.
+Also, note that if you're using inputs in your modal – iOS has a rendering bug which doesn't update the position of fixed elements when the virtual keyboard is triggered. There are a few work arounds for this, including transforming your elements to position: absolute or invoking a timer on focus to try to correct the positioning manually. This is not handled by Bootstrap, so it is up to you to decide which solution is best for your application.
Page zooming inevitably presents rendering artifacts in some components, both in Bootstrap and the rest of the web. Depending on the issue, we may be able to fix it (search first and then open an issue if need be). However, we tend to ignore these as they often have no direct solution other than hacky workarounds.