v0.62.0 — native GTK4 window presents frames
The GTK4-hosted Linux backend now actually shows pixels: sizes the GtkPicture in its GtkFixed (a content-less picture requested 0×0 and was invisible) and presents every frame from the GdkFrameClock via go-gtk v0.2.4's AddTickCallback (was a single pre-map present). newsreader -window with GO_WIDGETS_GTK=1 renders natively on Linux end-to-end (Xvfb-verified). Completes native windowing on cocoa+win32+gtk, CGO=0.