HTML API: Defer applying updates until necessary.

[dmsnell]

When making repeated updates to a document, the Tag Processor will end
up copying the entire document once for every update. This can lead to
catastrophic runtime behavior in the worse case.

However, when batch-applying updates it's able to copy chunks of the
document in one go and only end up copying the entire document once
for the entire batch.

Previously the Tag Processor has been eagerly applying updates, but in
this patch it defers applying those updates as long as is possible.

Trac ticket: Core-60697

When making repeated updates to a document, the Tag Processor will end
up copying the entire document once for every update. This can lead to
catastrophic behavior in the worse case.

However, when batch-applying updates it's able to copy chunks of the
document in one thread and only end up copying the entire document once
for the entire batch.

Previously the Tag Processor has been eagerly applying updates, but in
this patch it defers applying those updates as long as is possible.

[swissspidy]

Setting version to 6.3 because it was ​indicated on Slack that this might have been introduced in [55706].

[dmsnell]

In 57805:

HTML API: Defer applying attribute updates until necessary.

When making repeated updates to a document, the Tag Processor will end
up copying the entire document once for every update. This can lead to
catastrophic behavior in the worse case.

However, when batch-applying updates it's able to copy chunks of the
document in one thread and only end up copying the entire document once
for the entire batch.

Previously the Tag Processor has been eagerly applying udpates, but in
this patch it defers applying those updates as long as is possible.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/6120
Discussed in https://core.trac.wordpress.org/ticket/60697

Props: dmsnell, bernhard-reiter, jonsurrell, westonruter.
Fixes #60697.
Follow-up to [55706], [56941], [57348].

[dmsnell]

Re-opening for back-port to 6.5.

Merged in [57805]
​https://github.com/wordpress/wordpress-develop/commit/dabde02a67ac82b5692ce8167f6054c51a42d199

[audrasjb]

Self assigning for 6.5 backport before RC2 commit freeze :)

[audrasjb]

In 57815:

HTML API: Defer applying attribute updates until necessary.

When making repeated updates to a document, the Tag Processor will end
up copying the entire document once for every update. This can lead to
catastrophic behavior in the worse case.

However, when batch-applying updates it's able to copy chunks of the
document in one thread and only end up copying the entire document once
for the entire batch.

Previously the Tag Processor has been eagerly applying udpates, but in
this patch it defers applying those updates as long as is possible.
Developed in ​https://github.com/WordPress/wordpress-develop/pull/6120
Discussed in https://core.trac.wordpress.org/ticket/60697

Follow-up to [55706], [56941], [57348].

Reviewed by swissspidy.
Merges [57805] to the to the 6.5 branch.

Props dmsnell, bernhard-reiter, jonsurrell, westonruter.
Fixes #60697.

[dmsnell]

Thank you @audrasjb !