This is a spin-off from bug 37873: there is no feedback/indicator for text with negative indentation in table cells. The same happens for the page or columns when the paragraph is defined with extreme values.
Solution 1 is a small opacity to show the text in the background; but that requires to configure this option to comply with bug 37873 and it doesnt' work for objects outside the page area. So better we show little markers like the triangles known from Calc. Those indicators could be presented with the ruler- or the ruler background changes itself. This change could also be part of the solution for bug 117009.