hx-swap-oob fragments are silently dropped when the triggering element is detached before the response arrives

[mecampbellsoup]

Version: 2.0.7 (mechanism unchanged in 2.0.9 — src/htmx.js is identical in the relevant functions)

Summary

When the element that issued a request is removed from the DOM before the response lands, every hx-swap-oob fragment in that response is discarded — even though its target exists in the live document. The main swap still applies (its target resolved at request time), so the page ends up half-updated with only a console htmx:oobErrorNoTarget marking the loss.

Mechanism

The rootNode threading was introduced in #2846 to make OOB swaps work inside shadow roots — this report is about an unintended consequence for a case that PR didn't consider. In swap():

const rootNode = swapOptions.contextElement ? getRootNode(swapOptions.contextElement, false) : getDocument()

handleAjaxResponse passes the triggering element as contextElement. For a detached element, getRootNode() returns the orphaned subtree's own root — not the document — so oobSwap()'s querySelectorAllExt(rootNode, selector, false) finds nothing and the fragment is dropped. composed: true (the global selector modifier) doesn't help: a detached element's composed root is still itself, since composed only crosses shadow boundaries.

Reproduction

Any UI where a region containing the trigger can re-render while a request is in flight — e.g. a row list that's replaced by one interaction while a row's own hx-get is pending:

1. Click an element with hx-get targeting #content, whose response includes <div hx-swap-oob="beforeend:#tabs">…</div>.
2. Before the response arrives, replace the clicked element's parent region (list.innerHTML = list.innerHTML reproduces it deterministically with a delayed response).
3. The #content swap lands; the #tabs fragment is dropped with htmx:oobErrorNoTarget, though #tabs is present in the document throughout.

We hit this in production as a ~15%-under-load failure (a resource panel rendering its document with no tab-strip entry) and confirmed the mechanism with a forced-detach harness: detachment alone flips the outcome, 5/5 each way.

Expected

An OOB fragment addresses the document the user is looking at. The trigger element's identity shouldn't decide whether an unrelated, present target is findable — especially since the drop is silent apart from a console error, and the same response's main swap still applies.

Suggested fix

Falling back to the document when the context element is disconnected preserves the shadow-DOM behavior #2846 added for live triggers:

const rootNode = (swapOptions.contextElement && swapOptions.contextElement.isConnected)
  ? getRootNode(swapOptions.contextElement, false)
  : getDocument()

We ran this patch against the forced-detach harness and a 20× contention battery: the drop disappears; connected-trigger behavior (including shadow-root resolution) is untouched. Happy to open a PR against dev with this and a regression test if the approach looks right.