Page Settings: the option "Custom" is displayed as "A4"
Go to "Layout > Page Settings...". In the section "Page Size", select the option "Custom" in the drop-down list.
Result: "A4" is displayed (instead of "Custom") as the selected option.
Config: MuseScore e4004d6 on Xubuntu 14.10.
Comments
Same if you simply change the height or width using the spinboxes. A simple matter of having "Custom" last in the list instead of first where the code expects it. See:
https://github.com/musescore/MuseScore/pull/1366
Fixed in 173e0acab2
Automatically closed -- issue fixed for 2 weeks with no activity.