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With OpenSimulator 0.7.6, there will be an optional setting in OpenSim.ini
[EntityTransfer] DisableInterRegionTeleportCancellation = true
where setting this to true will prevent the cancel button from appearing during inter-region teleport. This can be useful on small grids or standalones where teleport is unlikely to fail, as cancellation itself can sometimes fail (due to hard-to-fix issues with the teleport protocol), result in an avatar that cannot move or re-teleport.