OO.ui.BookletLayout.prototype.focusFirstFocusable doesn't find focusable elements very well. For example, it misses our own ButtonWidgets. It should at least add '[tabindex]' to 'input, select, textarea, button, object' when finding elements to check. There is some interesting related discussion at T69156#1022986 onward. (Spotted by Anomie at https://gerrit.wikimedia.org/r/#/c/224966/5/src/widgets/CapsuleWidget.js,cm.)
(By the way, the method is duplicated as OO.ui.IndexLayout.prototype.focusFirstFocusable, we should put it somewhere else. We have OO.ui.isFocusableElement in core.js, that looks like a good place.)