I have a task pane (desktop) Office add-in built with the latest VS 2015 template for Excel and Word add-ins. When I open the file, the add-in task pane is not visible. My users need to click a button on the toolbar to open it, which they complain about.
In addition, I save the document with the task pane expanded but it does not affect that behavior. I remember a previous version of the add-in manifest (when it was called app for Office) kept the pane open but I upgraded it so I can use custom tabs in the Office ribbon.
Can I make the task pane stay expanded when the document or worksheet load?
There is now a way to do this: Automatically open a task pane with a document. And a sample at: Office OOXML EmbedAddin
Similar question here: Office Web App - Show task pane on load
For now you'll have to choose between persistent always-open task panes and custom tabs/buttons.
来源：https://stackoverflow.com/questions/39379427/keep-office-add-in-task-pane-open-at-document-load