The legend width is hard coded to 150px. This causes problems for entries with long names and is compounded with long lists of entries by the scroll bar. While I appreciate that it should have an upper and lower bound, it shouldn't be hard coded. It seems the current value was set to lock the width of the colour palette. I would replace
width: 150px; (
kibana/src/ui/public/vislib/styles/_legend.less line 44) with
min-width: 150px; max-width: 250px;
Feel free to suggest different values for min and max width if you think these are wrong.