Created attachment 260259 [details]
WebKit Nightly screenshot of the bug. Look at the scrollbar position.
Version: r186963 WebKit Nightly
Corresponding fixed Chrome bug: https://code.google.com/p/chromium/issues/detail?id=462809
Steps to reproduce the problem:
1. Open http://plnkr.co/edit/bKBag3Tfs5QYvwywMk24?p=preview
2. Click the first "Click and window will scroll down" button in the pane on the right half of the window.
3. The contents of the iframe are scrolled to the bottom where the <p id="target"> is.
What is the expected behavior?
The scroll position of the main page should not change. It should stay at its original position at the top of the page.
(Note the position of the scrollbar in the attached Chrome screenshot.)
What went wrong?
The scroll position of the main page moves downward a bit.
Note the position of the scrollbar in the attached WebKit Nightly screenshot.
By comparison, in both OS X Chrome and OS X Firefox, the scroll position of the main page doesn't change.
Created attachment 260260 [details]
Chrome screenshot demonstrating correct behavior
For further comparison, MS Edge behaves the same as Chrome & Firefox.