So you're using MultiViewElements in your NetBeans Platform application. When the user right-clicks one of the tabs they're able to split the editor you've created for them. Here you see the split functionality in the Source tab:
Here you see the split functionality in the Design tab, i.e., in both cases the user right-clicks in the tab and then sees they can split the editor:
However, what if you do NOT want that functionality? In the Branding dialog of the application, look for MultiViewElement.Spliting.Enabled as shown below, then right-click on the word below, choose "Add To Branding" and change the value from 'true' to 'false'.
Then run the application again... and the split functionality is gone in the Source tab:
As well as in the Design tab:
All this and similar branding switches are documented here:
One of the switches in the page above is USE_MNEMONICS, which I discussed here.