Created attachment 358497 [details] Screenshot Steps to reproduce: 1. Construct a page with at least one element overflowing viewport: ``` <html><head><style> body { width: 100%; margin: 10px; } body::before { content: ''; position: absolute; width: 100%; height: 100%; background: #777; } </style></head><body></body></html> ``` 2. On Safari 12, go to Safari -> Preferences -> Websites -> Page Zoom, and scale the current page down (50% is best to reproduce the bug). 3. Set Safari to fullscreen. 4. Open the constructed page. 5. Split the screen with another window (or the devtools). 6. Drag the separator to give the Safari window enough width (>= 1100pt). 7. Drag the separator again to reduce the width (< 1100pt). 8. The page turns into bad layout, where the root element is half the width of viewport (when the zoom factor is 50%). Since tons of public sites does not use `overflow:hidden` for `<html>`, this bug can be easily reproduced.
<rdar://problem/47093395>