1723903 - Shadow dom breaks text selection of content inside it

Status: RESOLVED DUPLICATE of bug 1545367

(Core :: DOM: Selection, defect)

Description

[mmis1000]

Attachment: Xnip2021-08-04_14-34-05.jpg

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:92.0) Gecko/20100101 Firefox/92.0

Steps to reproduce:

1. make a shadowroot to contain elements
2. move a span that has line-height shorter that containing container into it
3. try to select text and don't end you selection on the top of text (the cursor changed to arrow)

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
  <span id="a">
<span>sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds</span>
  </span>
  <br><br>
  <span id="a">
<span>sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds sds</span>
  </span>
  <style>body {
  line-height: 200%;
}
span {
line-height: 16px;
}
  </style>
  <script>
    
const root = document.getElementById('a').attachShadow({mode: 'open'});

[...document.getElementById('a').children].forEach(it => {
  root.appendChild(it)
})
  </script>
</body>
</html>

Actual results:

The select end up end at top of the text block instead of under cursor if you did not hover text at end

Expected results:

The text should be selected normally

Comment 1

[mmis1000]

Attachment: Xnip2021-08-04_14-34-35.jpg (The good result, when elements are not in shadow root)

Comment 2

[mmis1000]

See also: https://news.ycombinator.com/item?id=28055551

The broken site: comment section of https://bugs.chromium.org/p/chromium/issues/detail?id=1065085

Comment 3

[BugBot [:suhaib / :marco]]

The Bugbug bot thinks this bug should belong to the 'Core::DOM: Selection' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.