Look at the right edge. It appears when the mouse is plugged in, disappears when it's not. Charlie and Joe didn't see it because they didn't have a mouse plugged in.
See mui/base-ui#4645
🤖 Fable explanation
Found it — the scrollbar-gutter: stable isn't in our CSS at all. It's injected by Base UI's dialog scroll lock. When the side modal opens, useScrollLock in @base-ui/utils sets scrollbar-gutter: stable plus overflow: hidden on <html> (node_modules/@base-ui/utils/esm/useScrollLock.js:32), and restores the original values on close. The point is to hide the page scrollbar while the modal is open without the content reflowing into the freed-up space.
Why only you see it: the property still only has a visible effect with classic scrollbars. On your machine (mouse plugged in, or "Show scroll bars: Always"), the scrollbar is a real ~15px column — when the modal opens, the scroll lock hides the scrollbar and the stable gutter keeps that 15px reserved, so you see an empty strip where the scrollbar was. Your colleagues on trackpads have overlay scrollbars with zero width, so the same code produces no visible change. Cross-browser and extension-independent because it's driven by the macOS scrollbar mode, not browser state.
Look at the right edge. It appears when the mouse is plugged in, disappears when it's not. Charlie and Joe didn't see it because they didn't have a mouse plugged in.
See mui/base-ui#4645
🤖 Fable explanation
Found it — the
scrollbar-gutter: stableisn't in our CSS at all. It's injected by Base UI's dialog scroll lock. When the side modal opens,useScrollLockin@base-ui/utilssetsscrollbar-gutter: stableplusoverflow: hiddenon<html>(node_modules/@base-ui/utils/esm/useScrollLock.js:32), and restores the original values on close. The point is to hide the page scrollbar while the modal is open without the content reflowing into the freed-up space.Why only you see it: the property still only has a visible effect with classic scrollbars. On your machine (mouse plugged in, or "Show scroll bars: Always"), the scrollbar is a real ~15px column — when the modal opens, the scroll lock hides the scrollbar and the stable gutter keeps that 15px reserved, so you see an empty strip where the scrollbar was. Your colleagues on trackpads have overlay scrollbars with zero width, so the same code produces no visible change. Cross-browser and extension-independent because it's driven by the macOS scrollbar mode, not browser state.