Crossfade is currently always applied when you move sound events onto other sound events. The undo command is indeed the only way to revert that. The ctrl/alt/shift keys all have a function when dragging events, so there is no simple way to implement a key shortcut to prevent the crossfade.
One solution would be to add an option to the Tracks Region Properties dialog, but you would need to toggle that option if you want to change behavior. Do you need to avoid the crossfade consistently, or just occasionally?
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