To open a Sheet window, right-click on the name of the Sheet in the tree and select Open... [Ctrl+O].

Use the 'Show / hide properties' button
toggles to open and close the right-hand panel of the project-tree, or Sheet panel. Thus, it may cause an opened Sheet underneath, to be partially invisible. Close the panel again to see the Sheet in full again. The panel shows a number of properties of the Sheet that you can set, such as its width, heigt, background colour, gridsize, a general description for the Sheet and all the Notes

If you want to read the contents of your Sheets, as shown in the Sheet panels, use the arrow-keys to move down the project tree, while the right hand panel is open. Do not use the mouse-pointer for clicking the Sheets will close the panel and open the window.
Depending on whether or not the Sheet window and the Sheet panel have been opened yet, single or double clicking have the following consequences:
click | Sheet panel | Sheet window | consequence of click | ||||
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single click | opened Sheet panel | opened Sheet window | bring up Sheet window | ||||
closed Sheet window |
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closed Sheet panel | opened Sheet window | do nothing | |||||
closed Sheet window | do nothing | ||||||
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double click | opened Sheet panel | opened Sheet window |
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closed Sheet window |
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closed Sheet panel | opened Sheet window |
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closed Sheet window |
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By the way, you can also open a Sheet window directly from the search results. Sheet names will show when you search using tabs Global or Sheets in the search screen. Double click on the Sheet name. Not double clicking but right clicking and then selecting 'Open...' will also open the Sheet, however in that case only its description and collection of Notes.

Beware that the Sheet that you opened from your search results, will be removed when you close the search window. To prevent this, you may minimize the search window. Click the window's minimize button
and open a new search window (CTRL-G).