This is fixed in the latest version and was caoused by the fullscreen.
MC-5126
Sometimes you can't move items to or touch buttons in the upper part of the creative inventory
Resolved
Invalid
0
Well if you fix it by forcing update, you're modifying Minecraft in some way that the bug occurs.
Any mod issues need to be addressed to the mod creator.
This site is for addressing issues related to the base unmodded MineCraft.