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You can now right click anywhere on a web viewer, and you'll see 'Inspect Element'. Click this and it will bring up the Safari JavaScript Debugger.
The debug method JavaScript_Debugging.html JavaScriptのデバッグ in the Fusion Reactor ToolBox https://splashdocs.atlassian.net/wiki/spaces/FusionReactor/pages/2202959893 FRToolBoxの概要 will work nicely with Safari's JavaScript debugger, and will pass along any log messages.
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