Opt out of restoring placeholders for a promoted TF navigation

[julianrubisch]

Bug Fix

Description

When navigating a part of the page using Turbo Frames, in case of promoting to a normal page visit, the placeholders would be replaced but never resolve again.

The reason for this is that the turbo:before-cache event fires on a regular visit, so placeholders will be restored in order to be put into the Turbo cache afterwards. Contrary to a regular page visit, though, the whole page body will not be replaced, so the placeholders remain unresolved.

This would have been an easy fix if the turbo:before-cache event would have exposed an event.detail pointing to something usable, like the targeted Turbo Frame. Since it does not, however, I have come up with the following solution:

1. in a callback to the turbo:before-fetch-request event, the requested Turbo Frame ID is stored in the sessionStorage
2. the restorePlaceholders callback triggered by turbo:before-cache will return early if such an identifier is present, then delete it

This could potentially lead to some race conditions, but since this should be tab-isolated, I think it's pretty safe.

Reproduction repository here: https://github.com/julianrubisch/futurism_turbo_frame_placehoder

[hopsoft]

One question... but LGTM. 🚀