1994763 - Copying multiple lines from `<details>` causes everything in it to be copied

Status: VERIFIED FIXED

(Core :: DOM: Serializers, defect)

Description

[David Shin[:dshin]]

Attachment: Minimum Reproducible Case

Discovered this while trying to copy CSSWG resolution chat log. Confirmed via mozregression --launch 2025-10-16 -a summary.html

STR:

1. Load attached testcase
2. Select & copy a single line of A
3. Paste it somewhere to verify its content
4. Select & copy multiple lines of A
5. Paste it somewhere to verify its content

Expected: Both Step 3 and 5 pastes what was selected visually
Actual: Step 5 pastes the content of the entire details element

Comment 1

[Henri Sivonen (:hsivonen)]

This is pretty bad as far as how user-facing functionality is impaired, but considering that details isn't that common, it seems excessive to say S2. Saying S3.

Comment 2

[Edgar Chen [:edgar]]

<details> use the shadow tree, https://searchfox.org/firefox-main/rev/b67c6262ecbe7d4c9385a1bd335fd7dd1d3f887d/dom/html/HTMLDetailsElement.cpp#94, this is an issue of serializing a shadow DOM selection. The way we compute the flatten tree ancestors and offsets for serializing seems wrong, https://searchfox.org/firefox-main/rev/b67c6262ecbe7d4c9385a1bd335fd7dd1d3f887d/dom/serializers/nsDocumentEncoder.cpp#1415-1420.

Comment 3

[Edgar Chen [:edgar]]

The issue is about the UAWidget, there is no issue on normal shadow DOM. nsContentUtils::GetFlattenedTreeAncestorsAndOffsets() ignores the shadowRoot for UAWidget while compute the ancestor, due to https://searchfox.org/firefox-main/rev/f9d8702e26624ab46a35bf6561a7c8143c6f246a/dom/base/nsINode.cpp#4166-4168, but it doesn't do the same while compute the offset. And https://searchfox.org/firefox-main/rev/f9d8702e26624ab46a35bf6561a7c8143c6f246a/dom/serializers/nsDocumentEncoder.cpp#1256 might also need to ignore the UAWidget.

Comment 4

[Edgar Chen [:edgar]]

Attachment: Bug 1994763 - Ignore UAWidget when computing the offset iin nsContentUtils::GetFlattenedTreeAncestorsAndOffsets();

Comment 5

[Edgar Chen [:edgar]]

https://treeherder.mozilla.org/jobs?repo=try&landoCommitID=171707

Comment 6

[Edgar Chen [:edgar]]

https://treeherder.mozilla.org/jobs?repo=try&revision=9553f5dbf9337bbfd1071cd27fd8a769349bb913

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/d7693f55e44a
https://hg.mozilla.org/integration/autoland/rev/9b38501a7c6f
Ignore UAWidget when computing the offset iin nsContentUtils::GetFlattenedTreeAncestorsAndOffsets(); r=masayuki

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/b27319d90e9a
https://hg.mozilla.org/integration/autoland/rev/68096a67ac9b
Revert "Bug 1994763 - Ignore UAWidget when computing the offset iin nsContentUtils::GetFlattenedTreeAncestorsAndOffsets(); r=masayuki" for causing mochitest failures @ test_htmlcopyencoder_details

Comment 9

[Sandor Molnar[:smolnar]]

Backed out for causing mochitest failures @ test_htmlcopyencoder_details

Backout link: https://hg.mozilla.org/integration/autoland/rev/68096a67ac9b3e8a90fe09003ebcc39b2cf95906

Push with failures

Failure log -> TEST-UNEXPECTED-FAIL | dom/serializers/tests/mochitest/test_htmlcopyencoder_details.html

Comment 10

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/a5fa838aeac5
https://hg.mozilla.org/integration/autoland/rev/d384019d54d8
Ignore UAWidget when computing the offset iin nsContentUtils::GetFlattenedTreeAncestorsAndOffsets(); r=masayuki

Comment 11

[Cristian Tuns]

https://hg.mozilla.org/mozilla-central/rev/d384019d54d8

Comment 13

[Dana Lucaci, Desktop QA]

Reproduced with Firefox 146.0a1 (2025-10-16) on macOS 15 ARM.
Verified as fixed with Firefox 150.0a1 (20260303095635) and Firefox 149.0b3 (20260302093328) on macOS 15 ARM, Windows 10 and Ubuntu 22.04.