In Tab View, if you go to Source, drop down the menu, and RE-select your current sheet, your entire layout (that perhaps you worked a very long time on 
) completely disappears and defaults to as if you were starting anew.
Example scenario in which the bug occurs: checking your tab settings to make sure you had the correct sheet selected, and clicking on the sheet name again (as opposed to just clicking outside the box to cancel the drop-down action)
Command-Z sometimes works to restore the layout, but hasn’t worked consistently when I reproduced the bug several times. Regardless, reselecting a current selection should not cause this behavior.