Data, Format, Bookmar, Selection, Sync slicer ribbon tab buttons are toggle buttons. Perf. analyzer button doesn't work like a toggle button, while it does the same as the other buttons - shows/closes a pane on the right side of the window. Looks like a (minor) bug.
V then Close and X button on a pane header both remove the pane
Not necessary to have duplicate buttons for the same action. "Collapse" is a confusing name, it hides the pane, not collapsing groups on the pane.
Remove V button at all, replace it with > button and make the entire pane title clickable, so it's easier to hide a pane in one click (w/o clicking on a small pane icon on the toolbar).
Also, I don't think it makes any sense at all to remove icons from the toolbar as long as the toolbar remains on the screen. Keep the entire toolbar with all 6 icons (Data, Format, Bookmar, Selection, Sync slicer, Perf. Analyzer) always visible (like the too bar on the left is always vsible with report/data/model view icons). Make ribbon Data, Format, Bookmar, Selection, Sync slicer, Perf. Analyzer toggle buttons to show/hide corresponding panes, but not to remove icons from the toolbar. No need to have 'close' action for the panes anymore. We can just hide them.