This is expected. The way we center the dialog is via `top: 50%; transform: translateY(-50%);` The site overwrites `top` to `1.5em`, so that `top: 1.5em; transform: translateY(-50%)` causes the display issue. `Chrome`'s behaviour is known to be different than ours, they don't use `transform` at all and they have a hack in their code which basically hardcodes the value of `top`. The fix they did also looks correct. The issue is for some reason it's not applied, which is something that I don't know. There's another issue though. `<dialog>` is enabled on Nightly, so simply testing `Firefox/83` isn't sufficient.
Bug 1645046 Comment 19 Edit History
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This is expected. The way we center the dialog is via `top: 50%; transform: translateY(-50%);` The site overwrites `top` to `1.5em`, so that `top: 1.5em; transform: translateY(-50%)` causes the display issue. Chrome's behaviour is known to be different than ours, they don't use `transform` at all and they have a hack in their code which basically hardcodes the value of `top`. The fix they did also looks correct. The issue is for some reason it's not applied, which is something that I don't know. There's another issue though. `<dialog>` is enabled on Nightly, so simply testing `Firefox/83` isn't sufficient.