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This is a known issue with the initial release of Revit 2019. Autodesk made some internal changes to how windows work in order to allow windows to be torn off and placed on other screens in 2019; in the process it broke something in the dockable pane system such that addins using dockable panes would display this behavior in certain situations. Autodesk fixed this issue in 2019.1 so upgrading Revit to 2019.1 or newer should fix this issue.