Bug 174308 – position:fixed inside overflow:hidden container is incorrectly clipped

• Component: Layout and Rendering
• Status: RESOLVED CONFIGURATION CHANGED

[Javi]

Created attachment 314970 [details]
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.

[Javi]

Created attachment 314971 [details]
no painted button

[Simon Fraser (smfr)]

Created attachment 315001 [details]
Reduction

Smaller reduction attached. This is about position:fixed inside an overflow:auto, which WebKit/Chrome clip incorrectly.

[Ahmad Saleem]

I am not able to reproduce this usng attached ‘reduction’ in Safari 16.3, WebKit ToT (261811@main) and it is matching with other browsers (Chrome Canary 113 and Firefox Nightly 113). Just wanted to update.

Appreciate if someone else also can confirm so we can close this bug. Thanks!

[Simon Fraser (smfr)]

I believe this is fixed now.