Gkkt!
Chris, in the game, the kobold had a 19. With cover, that's a 17, which still hits. With your staff, it doesn't.
In the PHB, it calls for the attacker to pick a corner of its square and draw lines to each corner of the defender's square.
In this scenario, running along a wall does not constitute being blocked.
In the DMG, it calls for the defender to determine cover by drawing a line from a corner of its square to a corner of the attacker's square.
In the beginning "cover" text of the DMG, the wording is less precise than the PHB, however, the DMG also contains a "Getting Technical" section that has very specific, very precise rules for determining cover in melee.
Using those rules, you once again have cover.
To further support that case, the DMG replicates the PHB text when it comes to determining cover for ranged attacks.
Icing on the cake: both books describe fighting around a corner as an example of using cover (imprecise, but flavor).
After looking at the evidence, I'm going to give you cover.
I'm going to modify my post (which you may not have read).
If you want to use your staff to block, that is ok.
For the rest of you watching, D&D 4th ed. can still be rules-intensive.
