MacBook Pro 2018 & LG UltraFine 5K Display Power Comsumption I'm looking to purchase a new MacBook Pro and a monitor, and I'm currently looking at the 15" MacBook Pro (2018) with the LG UltraFine 5K Display.

I'm fully aware that the MacBook Pro and the LG UltraFine 5K Display are compatible with each other, as indicated by this Apple Support page. 

Having said that I noticed that the 15" MacBook Pro ships with an 87w power supply, whilst the LG UltraFine 5K Display provides up to 85W of charging power via the USB-C input.

Does that mean, that if the 15" MacBook Pro was performing some form of heavy computation, such as video rending, it could start to use the additional power requirements from the battery, and thus require the original charging cable to give it sufficient power (obviously at some point the battery will go flat)?

I'm probably looking into this far more than what is required, but it does seem strange that the latest 15" MacBook Pro laptops require more power than the "official Apple supported" monitor can provide.
For MacBooks (including Pro), if the requested voltage based on current system load is higher than the power available, it will try to make up for the difference by drawing from the battery; if the requested voltage is lower than the available power, it will use the extra power to recharge the battery until it is full. 

For using the 85W charger on an 87W MacBook Pro, the difference is so small that it is virtually unnoticeable. In rare cases, your battery might be losing a small amount of power slowly when the machine is under max load. In most case, it will be just fine.

Note: The LG UltraFine 5K is made for MacBook Pro 15" and MacBook Pro 13", and you can see that MacBook Pro 15" has the best feature support. So it is safe to assume that everything, including the power delivery over USB-C, is highly optimised.