New to intellij plugin development.
It's hard to find an open source codebase on github short of downloading all the plugins, which isn't relaly feasible. None of their plugin pages ever has a screen shot of what the plugin looks like - oh well.
Simple enough question, can an intellij plugin with a ui window have, within that window, it's own tabs? Is there any short sample code for this, or a link to a github account?
An IntelliJ plugin can have any UI it wants; there are no limitations.
If by "UI window" you mean a toolwindow (a panel on the side of the screen), you can use toolWindow.getContentManager().addContent() to create tabs in that toolwindow. You can search for usages of this method in the IntelliJ IDEA Community Edition codebase to find plenty of examples of its use.