See bug 1559192 - XUL deck layout frame, which is <deck> and <tabpanel> is on the cutting list. There's very few <tabpanel> usages outside tests, in Firefox: https://searchfox.org/mozilla-central/search?q=%3Ctabpanels&path= For mail: https://searchfox.org/comm-central/search?q=%3Ctabpanels&path=%5Email&case=false®exp=false For calendar: https://searchfox.org/comm-central/search?q=%3Ctabpanels&path=%5Ecalendar&case=false®exp=false
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See bug 1559192 - XUL deck layout frame, which is <deck> and <tabpanels> is on the cutting list. There's very few <tabpanels> usages outside tests, in Firefox: https://searchfox.org/mozilla-central/search?q=%3Ctabpanels&path= For mail: https://searchfox.org/comm-central/search?q=%3Ctabpanels&path=%5Email&case=false®exp=false For calendar: https://searchfox.org/comm-central/search?q=%3Ctabpanels&path=%5Ecalendar&case=false®exp=false