Depending on the current PC setup of the user and the data loaded in the Grid it's possible to observe such behavior in some cases when the
user tries to open given cell for editing faster than the browser is able to close the last edited cell (as each operation requires traversing and modifying of the DOM).
The "Navigatable" option of the Grid adds additional layer of complexity by adding more event handlers to the Grid rows/cells in which the current element is selected and previous selection is removed based on complex logic. That why turning it off improves the Grid performance which on other hand prevents the user from reproducing the previous behavior.
In current case I would suggest to try reducing the page size of the Grid or use "InLine" / "PopUp" edit modes.
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