Bug 304726 – REGRESSION (Safari 26.2): Scrollbar drag fails on first attempt after dynamic DOM replacement in overflow container

• Component: Scrolling
• Status: RESOLVED FIXED

Created attachment 477858 [details]
Minimal test case to reproduce the bug

## Problem
  Scrollbar drag completely fails on first attempt after dynamic DOM replacement inside an overflow container. The thumb does not move at all. Second attempt works normally. Mouse wheel scrolling always works. Both vertical and horizontal scrollbars are affected.

  ## Steps to Reproduce
  1. Create a container with overflow: scroll (or overflow-x/overflow-y)
  2. Replace inner DOM content via JavaScript (e.g., innerHTML)
  3. Immediately try to drag the scrollbar thumb

  ## Expected
  Scrollbar drag works on first attempt.

  ## Actual
  First drag fails (thumb doesn't move at all). Second drag works.

  ## Regression
  - Safari 26.1 (21622.2.11.11.9): Works correctly ✅
  - Safari 26.2 (21623.1.14.11.9): Bug occurs ❌

  ## Test Case
  See attached: webkit-bug-testcase.html

[Radar WebKit Bug Importer]

<rdar://problem/167249048>

[Simon Fraser (smfr)]

I can reproduce even on first load of the attachment.

[Simon Fraser (smfr)]

Pull request: https://github.com/WebKit/WebKit/pull/56239

[EWS]

Committed 305383@main (bc68ba9c807c): <https://commits.webkit.org/305383@main>

Reviewed commits have been landed. Closing PR #56239 and removing active labels.