A Sheet window and a Sheet panel are designed to overlap to save room on your screen. The panel is the right-hand part of the project tree, that will show the details of the node that is selected in the tree. The window is the visualization of a Sheet that was clicked in the tree, which is also positioned next to the tree. The panel can be toggled by the 'Show / Hide properties' button . The window can be closed normally by the 'close' button .
The Fit to Screen button in the middle of the upper icon bar resets your project-tree and the latest Sheet you opened. If you opened more than one Sheet, the latest one opened will be on top and hide the others underneath.
If you wish to see all opened Sheets, click the tile button .
This is a toggle-button, so clicking once opens a panel (not a window) to the right of the tree, and clicking it again closes it. The panel shows you and lets you edit the properties of the item that is highlighted in the tree. To show the properties of Attachments and Notes, you can click them after you opened the panel. But if you click on a Sheet, the Sheet opens as a new frame where you cannot change properties of the Sheet itself. If you want to set the properties of the Sheet, use the arrow keys to move to the Sheet you wish to highlight.
To illustrate the above, here is an example of a Sheet that is opened as a window that is at the same time partially overlapped by the same Sheet's properties in the panel on the right of the project tree.