It looks like rendering is broken for 2.14.4 and 2.15 on high DPI (HiDPI/Retina) displays when GNOME/GTK scaling-factor=2 is set: The contents of the web views are appearing blank - text is not rendered, however mousing over it does cause the cursor to change. This is under Geary master, where we are also setting WEBKIT_DISABLE_COMPOSITING_MODE=1 up the top in main().
FYI Geary bug here: https://bugzilla.gnome.org/show_bug.cgi?id=778465
*** This bug has been marked as a duplicate of bug 168128 ***