`flex-container` breaks `text truncate`

[wxiaoguang]

It seems to be a regression of "Reduce margins on user settings page, introduce flex-container (#26046)"

And maybe it's also related to "Reduce margins on admin pages #26026", which started using "display: flex"

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With flex-container (display: flex):

Without flex-container:

[wxiaoguang]

@silverwind would you like to take a look?