The exit-frame self-check compared the y label strip to the pixel and printed a
FAIL at GDK_SCALE=2 where it measured 174 -> 172. Measured (temporary probe,
reverted): the strip is `widest major tick label + padding`, and the tick step
follows the axes length, which the resize changes -- 18 majors over 1440 device px
with "-800m" widest, 8 over 1080 with "-500m", 78 vs 76 px in the bitmap font. The
layout was recomputed; the equality was the wrong assertion. The check now asserts
what actually distinguishes a recompute from a scaled stale layout: the axes and
its strips tile the *current* viewport, and the strip is nearer the previous strip
than that strip scaled by the resize ratio (2 px of drift against a scaled 118).
`on_exit_timer`'s main-context pump can re-enter its own 20 ms timeout, which ran
the exit block twice in one observed run; `on_teardown` is guarded the same way so
the producer join, the window destruction and the quit happen exactly once. A window
closed by hand takes the same ordering through `GtkWindow::close-request`, which
`gtk_window_destroy()` does not emit.
Martino Ferrari
377cca8e9b