So you use the results of the display engine to display the same stuff
differently, is that right?
If so, I'm not sure I understand how to do it "properly". The Emacs
display engine performs the layout calculations (which determine, for
example, where each screen line ends and the next one begins, or what
stuff fits into a window) by examining each character of buffer text
in turn. Without this examination, how can we know how to display the
text as SVG? And if we use the current code to produce this
information, then what did we gain?
I'm probably missing something.
Do you notice the popup in the top window in the image attached in OP? It has clean lines over buffer content with mixed (fixed + variable pitch) fonts. Is it possible to have that in graphical Emacs? Precise positioning of popups can open up graphical Emacs to immense possibilities like feature filled tooltips, inline completion popup, multi-column layouts, watermarks etc.