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Create a <timepicker> in 12-hour format. Set the timepicker time to some time after noon (i.e., PM) if it isn't already. Highlight the hours field of the timepicker, and type a number less than 12 to replace the value there. Note how the timepicker flips from PM to AM. It shouldn't do that.
This appears to be related to the issue of the order of onchange event handlers being fired, described in https://groups.google.com/forum/?fromgroups=#!topic/mozilla.dev.tech.xul/lOvK6bAwVoI . It appears that the problem described above only happens when you have an onchange event handler that fetches the value of the timePicker. If you use the strategy outlined in that thread to defer your onchange handler until all others have been called, the problem appears to go away.
Moving to Core:XUL per https://bugzilla.mozilla.org/show_bug.cgi?id=1455336
Component: XP Toolkit/Widgets: XUL → XUL
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