One thing to mention since this came up during an estimation review, is that currently sidebars don't show up in popup windows. Per Gijs who did a bit of digging "We hide pretty much all the buttons from the toolbar to keep things minimal (but kept the hamburger menu for access reasons to e.g. print). However, you can still try to use the keyboard shortcuts, and the View > Sidebar menu on macOS. The menu entries are not disabled, and indeed the checkmark in front of the items appears/disappears, but to no avail (ie the sidebar never becomes visible). I guess in an ideal world we fix this so the sidebar menu is disabled and the shortcuts no-op, but it's probably not critical given the existing state."
Bug 1892012 Comment 1 Edit History
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One thing to mention since this came up during an estimation review, is that currently sidebars don't show up in popup windows. Per Gijs who did a bit of digging "We hide pretty much all the buttons from the toolbar to keep things minimal (but kept the hamburger menu for access reasons to e.g. print). However, you can still try to use the keyboard shortcuts, and the View > Sidebar menu on macOS. The menu entries are not disabled, and indeed the checkmark in front of the items appears/disappears, but to no avail (ie the sidebar never becomes visible). I guess in an ideal world we fix this so the sidebar menu is disabled and the shortcuts no-op, but it's probably not critical given the existing state." Edit: Let's tackle this in bug 1895868