How do I customize the controls?

I'm using shift + click in creative mode to place items on the side of hoppers, furnaces, etc., but since I'm flying, my player moves downward.

I looked at the in-game settings, but there is nothing about this command. I looked for a config file, but the only one I found (at .minecraft/options.txt) is limited to the same options from the in-game settings menu.

Is there a way to customize the controls used for placing items on the side of interactive blocks or for flying?

To change the minecraft controls you can go into the controls menu and edit them to your liking.

To place a block against the side of a block with a gui such as a hopper or furnace, without sinking, press shift and space while clicking.

If that is too many buttons for you, try using an autohotkey script. https://autohotkey.com/ That will allow you to create custom hotkey combinations so for example you can press shift and it will automatically press space at the same time. The problem with this solution is that it will still press shift and space when you are on the ground, resulting in constantly jumping up and down while you press shift. It's possible you could instead use the macro keybind mod. This may allow you to only do the shift+space combo when you are not on the ground. The problem with both of these solutions is that you will no longer be able to go down while flying, without double tapping space to disable fly mode. http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/1275039-macro-keybind-mod

Go to Options/Controls. Find the control called 'Sneak. Click on where it says 'LSHIFT' and press a key. This will reassign sneak to that key, so you can use shift to place blocks on hoppers without descending when flying. When you do want to descend, then press the key you chose. Remember not to assign any keys already used, if you do, it will show up in red. I use the 'K' key.