You can change what happens when you click on a link in a diagram so that it opens in a new tab or the same tab in draw.io for Confluence Server.

1. Edit the page with the diagram in it, select the draw.io diagram macro (in edit mode), then click on Edit Macro.
   
2. In the draw.io Macro Settings dialog, select a different option from the Links drop down list, then click Save.
   
3. Save and publish your page.

Change the link behaviour in draw.io instance-wide

You can customise draw.io so that links in all of the diagrams in your instance open in another browser tab by default.

1. As an administrator, click on the gear in the Confluence menu, then select General configuration to go to your instance’s administration area.
2. In the left panel, under the draw.io add-on heading, select Configuration. Then select the Configuration tab.
3. Add "defaultMacroParameters": { "links": "blank" } to the UI Configuration field, then click Save changes.
   

See how to customise draw.io for Confluence Server