[Webkit-unassigned] [Bug 174308] New: Painting issue when using dual/mirrored screen

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=174308

            Bug ID: 174308
           Summary: Painting issue when using dual/mirrored screen
           Product: WebKit
           Version: Safari 10
          Hardware: Macintosh
                OS: macOS 10.12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: javijoomla at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 314970

  --> https://bugs.webkit.org/attachment.cgi?id=314970&action=review

html to reproduce the issue

# Summary
Our automated visual test suite highlighted a screen painting issue in webkit. A button that was painted before suddenly stopped being painted in the browser in special conditions. See the following snapshot: https://drive.google.com/file/d/0B_fG9e25GNY7TFdQV2k3cEV5RHc/view?usp=sharing

# How to reproduce
We were able to replicate it when using a mirrored external screen, or if we used a dual screen, only when moving the Chrome window to the second screen (see video: https://www.youtube.com/watch?v=gTONU76r0os&feature=youtu.be).

Our automated test suite is ran by Travis on a Linux Ubuntu machine and got the same issue.

We have reduced down the HTML/CSS to the critical skeleton, and identified 3 potential rules that create the breakage (see html file attached).

To reproduce:
1- open the file attached in a Chrome or Safari browser.
2- Change the window height. Reduce it enough until there is not enough space for the button to appear under the image and has to float over the image.
3- make sure you have two displays connected to your computer. Move the browser window from on display to the other. You will see that suddenly the button disappears. 

# URL
use the file attached

# Platform and OS
- Apple macOS Sierra 10.12.5
- Safari Version 10.1.1 (12603.2.4)
- Google Chrome Version 59.0.3071.115 (Official Build) (64-bit)
- Google Chrome CanaryVersion 61.0.3150.0 (Official Build) canary (64-bit)

It is worth noting that when inspecting the DOM while the content has "disappeared", the browser still think the HTML elements are in the correct position (see https://drive.google.com/file/d/0B_fG9e25GNY7TFdQV2k3cEV5RHc/view?usp=sharing), hence our belief this is a painting issue with the browser itself.

This bug suddenly arose on Wednesday, July 5th. The logic in our code was introduced on May 31st.

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