What does the blue bar in the experience bar mean?

I noticed this bar for the first time yesterday after being offline for a few days. I'm fairly close to levelling up, and the usual grey bar rest xp indicator is completely full i.e. all the way to the edge of the screen.

My suspicion is that the blue bar is indicating the 'rest xp' that has accumulated beyond your current level.

E.g. if you're currently level 34 and 90% of the way to level 35, your xp bar would be completely full to the end with the 'grey bar' representation of rest xp, and this blue bar is indicating what percentage of the xp from 35-36 is being covered by rest xp.

Possibly this means that rest xp is capped at the amount for you to level up once plus one full extra level, which does sound plausible.

A quick Google search told me it's a "rested" bar. When you log out, it will fill up so that when you log back in you will start to gain xp at a faster rate. There is a similar mechanic in WoW, for reference.

EDIT: As wondercricket pointed out, I was thinking of the grey bar. I will leave this answer here in case anyone looking for if finds this post first.