Both renderers placed the baseline at box_top + height/2 + descent/2. Centring
glyphs of visual height ascent+descent in a box of height H puts the baseline at
box_top + H/2 + (ascent-descent)/2; the two agree only when ascent is exactly
twice descent. DejaVu is nearer 4:1, so labels sat high against the top edge -
measured at 5px above and 11px below for a 14px button.
ButtonText also sized itself from the nominal font size, which is smaller than
the text's visual height (17px of ink for a 14px DejaVu font), leaving the
button too short to centre its label in. It now measures ascent+descent, with a
fallback for font objects that cannot report metrics.
docs/images/example_07_pressed_state.png was stale - no example writes it, the
demo emits demo_07_pressed.png at the repo root and the docs copy had been
placed by hand in November. Refreshed here; the demo should write straight to
docs/images/ so it cannot drift again.
dtourolle
