Here is a text node you can't select while the dialog is open. I'm selectable.

Summary

Harness status: OK

Found 1 tests

Details

ResultTest NameMessage
PassTest that inert nodes cannot be selected. The test passes if the only text you can select is inside the dialog.
Asserts run
Pass
assert_equals("I'm selectable.", "I'm selectable.")
    at Test.<anonymous> ( /html/semantics/interactive-elements/the-dialog-element/inert-node-is-unselectable.html:17:3)