Nested multi-column layout problem in Safari/WebKit

[MurakamiShinyu]

WebKit (Safari) has known issues on nested multi-column layout:

• [css-multicol-1] Nested multi-columns are laid out incorrectly
• Nested multi-columns can cut off text

Due to these issues, when using Vivliostyle with Safari/WebKit, nested multi-columns layout may be broken, or page breaking inside non-root multi-column boxes may not work well.

See also the following PR comment:

• fix: Improve page/column breaking using browser multi-column feature #1571 (comment)