Ctrl + Q doesn't work if you use a mouse button as Q keybind.
What I expected to happen was...:
Drop a stack of items with ctrl + "Button 4" when I am into my inventory in creative or survival gamemode
What actually happened was...:
It only works if the keybind is a keyboard key. It doesn't work for mouse buttons
Steps to Reproduce:
1. Change your "selected drop item" keybind to any mouse button except left and right click, for example "scroll up", "scroll down", "middle button", "button 4", etc.
2. Have any stackable item on your inventory or hotbar
3. Open your inventory and try Ctrl + (the new mouse key) to drop more than one item
Related issues
Comments

Thank you for your report!
We're tracking this issue in MC-577, so this ticket is being resolved and linked as a duplicate.
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This duplicates MC-577