Here's a title pane with an arrow button to hide and show the contents. Activating the button will toggle the display of the content pane. When the button is activated, focus remains on the arrow button. Press the down arrow to set focus from the arrow button to the open content pane. The contents pane is put into the tab order when the pane is made visible so pressing tab from the down arrow button will focus the contents pane when it is visible.. The button action is described via the title attribute which is updated depending upon the visibility of the content. The title for the button appends "Hide" or "Show" to the tile of pane.
Currently the contents pane is given a role of region and has a labelledby property which points to the title. The region role is not yet supported by screen readers.