In order to make easy to understand, elegant GUIs one must limit what they can do. Sprengui has these limitations on purpose. Some of these decisions may be unusual, but the end-user experience is improved because of them.
Tabs should always be vertical and groupable. This allows for easy organization and scrollable tabs without reducing visibility of tab identifiers. Tabs should always be located on the left and auto-collapse when the user is not interacting with it.
Depending on the state of the application you may want a scrollable document. This should take up the majority of the screen (besides the tab pane and the header menu). This should always be the pane in the lower right of the screen so that the mouse can hit the edge to grab the scroll bar. If the scroll pane primarily scrolls horizontally timewise that should be the default action of the scroll wheel. There should only be one scroll view, and the only other thing that scrolls is the tabs pane. Nested scroll views are bad for obvious reasons, so disallowed. Natural scrolling should be preferred for touchpads and touchscreens, but not mouse wheels.
Preview panes appear above the scroll view (or may use an additional monitor). Useful for things like video editing, picture editing, IDEs and 3D modeling.
The header menu reserves the middle space for dragging the window and displaying the file name. The header menu may have either a tabs button or a back button (they may display the same "<" icon, though). Followed by the back icon, is a search icon (which may expand to fill up the entire header menu except for the back and exit buttons when clicked). On the right there is a menu icon which shows a vertical list of additional pages.
Mode switchers are floating action button(s) that are located on the right side of the screen on top of the scroll bar. A mouse clicked on the very right edge of the screen will press a button. This will change into selecting the scroll bar if dragged. It can be assumed that you can have at least 8 mode buttons without overflowing the screen, so to never overflow, the number maxes out at 8.
The Action Tray is located at the bottom of the screen. It resembles the mode switcher, and works the same way with the scroll bar. The action tray should give you buttons for what operations you can do in whatever mode you are in. It can be assumed that you can have at least 8 actions buttons without overflowing the screen, so to never overflow, the number mabxes out at 8.
The context menu has quick actions that are available in all modes, but may change depending on where they are openned. Stuff like undo, redo, select all, cut, copy, paste belongs in a context menu. Additionally may have an "object properties" or similar option. Context menus are openned with either right click, menu key (on some keyboards) or two-finger tap.
Pop-ups aren't visually pop-ups in the traditional sense. They are equivalent to pages that you can return from. They can't have tabs because you have to be able to go back. You can have an action tray, mode switcher and hamburger menu, though.
The document is located inside of the scroll view. It may have rasters, text, buttons, etc.