The <<.def tv-auto-open-on-import>> [[variable|Variables]] controls whether the `tm-import-tiddlers` message causes the tiddler [[$:/Import]] (which lists the pending imports) to open in the story river.
By default, the tiddler is opened. But if this variable has the value <<.value no>>, it isn't.
An example of setting this variable to <<.value no>> can be found in the upgrade plugin within the [[TiddlyWiki Upgrade Wizard|https://www.tiddlywiki.com/upgrade.html]]. People can drag their wiki files onto the wizard without triggering the normal import display.