Error occurs by links to external site in TOC

[MurakamiShinyu]

Describe the bug

With bookMode=true, when TOC (e.g., the <nav> element) has links to external site, the CORS error occurs:

"Received an empty response for https://.... This may be caused by the server not allowing cross-origin resource sharing (CORS)."

To Reproduce

Test Vivliostyle Viewer with
https://vivliostyle.org/viewer/#src=https://vivliostyle.org/&bookMode=true&renderAllPages=false

Expected behavior

In the Book Mode, multiple web pages in a site are expected to be loaded consecutively by the links in the TOC, but external sites should be excluded.

Screenshots

[MurakamiShinyu]

Test with the version the bug is found:
https://vivliostyle.github.io/viewer/v2.1.1/#src=https://vivliostyle.org/&bookMode=true&renderAllPages=false

Test with the latest canary version:
https://vivliostyle.now.sh/#src=https://vivliostyle.org/&bookMode=true&renderAllPages=false