That is a bit tricky to reproduce it.

Steps:
Add 4-5 custom colors to popup.
Select white (i mean rectangle at position 0,0); -> predefined palette popup will be closed, color selected - as usually.
Open popup again (just click on color picker control).
Move mouse on any rectangle in palette.
Press left/right/up/down.

Don't work.

After some number of pressings, work of keys will be restored.