When creating VSTO documents that use an Action Pane, the Action Pane gets "lost" if you open another non-VSTO document. The Action Pane is hidden by the different document (as it should), but when you switch back to the original VSTO document, the Action Pane does not automatically re-open itself. This can be fixed by handling the ThisWorkbook_WindowActivate event. In this event we can check the state of the Actions Pane and re-display it if necessary:
private void ThisWorkbook_WindowActivate(Microsoft.Office.Interop.Excel.Window Wn)
{
if (!Globals.ThisWorkbook.ActionsPane.Visible)
{
Globals.ThisWorkbook.ActionsPane.Visible = true;
}
if (!ThisApplication.DisplayDocumentActionTaskPane)
{
ThisApplication.DisplayDocumentActionTaskPane = true;
}
}