I don't know if this is a bug or not. Every time I open MuseScore to create a score the Palette appears over the score not next to it as it used to. Also when I play the score, having moved it away from the Palette, it hops back behind the Palette.
I am running Windows 7.
Comments
If you double click the top bar of the palettes they should dock on the left side of the screen. If they don't stay there the next time you open MuseScore then try revert to factory settings, which should not be necessary.
You must have inadvertently undocked it. Simply drag it back to the edge until the docking icons appears and dock it again.
In the future, when asking for help using MuseScore, please use the Support forum.