I'm a little confused about how to use undoManager in Firefox. According to Mozilla, Firefox 20 now has support for it, but I can't figure out where. I've checked under document
, Objects
, etc., but can't find it anywhere. I'm sure I'm missing something obvious, so can anyone help?
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Daniel H
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You have to set the "dom.undo_manager.enabled" preference to use it, so far, since the spec is still in flux.

Boris Zbarsky
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Thank you very much! But I had to add the preference manually, so I'm wondering why it wasn't just included and set to disabled by default. Anyway, I'll have to look into this feature more because I also thought undoManager would access the current undo stack, which it doesn't seem to be doing... – Daniel H Apr 09 '13 at 19:07
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Does it actually work? I made a simple document with an input, textarea and contenteditable div, and each always has a null `undoManager` property and `document.undoManager.length` is always 0. – Tim Down Apr 26 '13 at 14:28
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`document.undoManager` should work at least for some things, since there are automated tests that it does... See http://mxr.mozilla.org/mozilla-central/source/content/html/content/test/test_undoManager.html – Boris Zbarsky Apr 26 '13 at 16:59