in model/src/main/java/jetbrains/jetpad/model/composite/Composites.java [415:427]
CompositeT firstFocusable(CompositeT v, boolean deepest) {
for (CompositeT cv : v.children()) {
if (!cv.visible().get()) continue;
if (!deepest && cv.focusable().get()) return cv;
CompositeT result = firstFocusable(cv);
if (result != null) return result;
}
if (v.focusable().get()) return v;
return null;
}