isBlank function
Whether frame would draw nothing at all.
Emptiness measured in what a reader sees rather than in characters. A frame
of nothing but escape sequences — the ESC[0;0H that trails a screen
clear, a colour the child set and then wrote nothing in — settles as a row
that paints blank, and a pane has a fixed number of rows to spend. A blank
line the child wrote itself has always been dropped here for that reason;
this only stops the same row costing more when it arrives dressed up.
Implementation
bool isBlank(String frame) => stripAnsi(frame).trim().isEmpty;