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When surpassing a certain number (somewhat around 15-20) of line breaks (maybe other characters too, not sure) in a notebook the entire content of the page including the header jumps some 10px to the left. It does not matter whether those line breaks are within one cell or distributed across several cells. Afterwards, the cursor disappears when inside text content while the cell still accepts further input. The cursor can still be positioned although it will not get rendered anymore.
In my browser this happens for this cell. Removing one of the trailing blank lines has the content jump back and the cursor re-appear.
Reproducible in Chrome (72.0.3626.81)
Ok in Firefox 65.0
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