ToolWindow.setSplitMode() not working until after click?

I'm trying out the basic `CalendarToolWindowContent` example. I finally realized after about an hour `secondary="true"` on the toolWindow xml (in plugin.xml) was overwriting the setSplitMode() I was calling. It was only after clicking on the side-button that the setSplitMode(false, null) would then take effect and no longer be split. Why is there no function like `ToolWindow.setSecondary()`? This is even more confusing, because the docs for ToolWindowEP dont have documentation on the secondary attribute. Is this expected behaviour?