Fix JavaScript unit test regressions from recent page loading changes.

Review Request #13295 — Created Sept. 29, 2023 and submitted — Latest diff uploaded


Review Board


Work was done for 6.0 beta 3 to delay rendering of pages until renders
all completed, in order to avoid page jumps. That work regressed some
unit tests, which were no longer testing the correct state.

This fixes those tests by better mirroring the body element and
relevant containers all within $testsScratch, resetting their styling,
and managing states while testing to ensure we're working with the
equivalent of the styles used while loading a page. It then tests the
newer visibility states expected during and after page load.

JavaScript unit tests all pass on Firefox and Chrome.