Select Clone Brush
Alt-click on image -- adds '+'
Click Add Global Source -- Brings up Sources panel with current layer in
Click on layer in Sources panel
Click on dustbin in Sources panel -- layer deleted
Click on eye in bottom left of Sources panel -- image disappears (shouldn't do this, I presume)
Change Source: on top context text menu to Global
Move mouse over image area -- goes all flickery! -- made up of squares from image.
Also:
Duplicating and deleting layers also leads to strangeness.
Even pressing control sets it off.
Source: on context menu sometimes gets greyed out. Restoring it to Current Layer seems to help.
Looks like a bit of code has lost its way.