Mouse Tweaks replaces the standard RMB dragging mechanic, adds two new LMB dragging mechanics and an ability to quickly move items with the scroll wheel.`

Tweaks

RMB Tweak

Very similar to the standard RMB dragging mechanic, with one difference: if you drag over a slot multiple times, an item will be put there multiple times. Replaces the standard mechanic if enabled.

Configuration setting: RMBTweak=1

Hold your right mouse button:

Drag your mouse around the crafting grid:

You can drag your mouse on top of existing items:

LMB Tweak (with item)

Lets you quickly pick up or move items of the same type.

Configuration setting: LMBTweakWithItem=1

Hold your left mouse button to pick up an item:

Drag your mouse across the inventory. Items of the same type will be picked up:

Hold shift and drag. Items of the same type will get "shift-clicked":

LMB Tweak (without item)

Quickly move items into another inventory.

Configuration setting: LMBTweakWithoutItem=1

Hold shift, then hold your left mouse button:
(Mouse cursor is not visible for some reason)

Drag your mouse across the inventory. Items will get "shift-clicked":
(Mouse cursor is not visible for some reason)

Wheel Tweak

Scroll to quickly move items between inventories. When you scroll down on an item stack, its items will be moved one by one. When you scroll up, items will be moved into it from another inventory.

Configuration setting: WheelTweak=1

Configuration setting: WheelSearchOrder=1

When you scroll up, the mod will search for items from last to first (when this is set to 1) or from first to last (when this is set to 0).

Configuration setting: WheelScrollDirection=0

Set this to 1 to invert the default scroll actions. So, when set to 1, scrolling down will pull the items and scrolling up will push the items.

Set this to 2 to enable the inventory position aware scrolling. Scrolling up will push the items into the other inventory if it's above the selected slot, or pull items from the other inventory if it's below the selected slot. Vice versa for scrolling down.

Configuration setting: OnTickMethodOrder=Forge, LiteLoader

Mouse Tweaks can use multiple APIs for an OnTick method that it requires. You can use this setting to control the API it prefers. This shouldn't really matter at all. If a method isn't supported (for example, you don't have the API installed) the mod will proceed to check the next ones.

Configuration setting: ScrollHandling=0

Toggles between "smooth scrolling, minor issues" (0) and "non-smooth scrolling, no issues" (1). When set to smooth scrolling, minor issues may be experienced such as scrolling "through" JEI or other mods. When set to non-smooth scrolling, those issues will not happen, but the scrolling will be a little non-smooth. Non-smooth scrolling works only with the Forge OnTick method.

This option is set to smooth scrolling by default because the aforementioned issues require rather specific conditions to trigger and aren't very impactful, while scrolling items is something you do all the time and want the experience to be as good as possible.

Old Installation Instructions

Spoiler (click to show)

1.6:

There's an excellent tutorial on how to install mods in Minecraft 1.6+, made by xisumavoid.

You can install it however you want.
For example, you can extract the Mouse Tweaks zip-file into your minecraft jar file (don't forget to remove the META-INF folder!) - this way it is compatible with LiteLoader (no matter what you put into your JAR first) and other mods that don't overwrite the same class file.
Also you can install one of these: ModLoader, Minecraft Forge, LiteLoader (or two of them at the same time) and drop the mod's zip-file into your mods folder. On 1.6+, the mods folder is situated either in your .minecraft folder, or in your .minecraft/versions/[your-version-folder] folder. Please note, however, that if you are using LiteLoader without anything else, you will need to either install Mouse Tweaks using the first way (extracting into the JAR), or change the extension of the Mouse Tweaks' zip-file from .zip to .litemod.

Spoiler (click to show)

Pre-2.0:

1. Install either ModLoader or Minecraft Forge.
2. Drop the mod zip file into your mods folder.

NoModloader

1. Open your minecraft.jar.
2. Delete the META-INF folder.
3. Open the Mouse Tweaks zip file.
4. Drag the class file from the Mouse Tweaks zip file into your minecraft.jar.

Compatibility

Mouse Tweaks is compatible with everything based on GuiContainer (as long as the behavior isn't changed too much).

If your GUI isn't based on GuiContainer, or if you want to improve compatibility (making Mouse Tweaks ignore some slot, for example), take a look at the API documentation.

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Feel free to include Mouse Tweaks in modpacks. If you want to list a download link to the mod, please use the CurseForge link so it counts towards the download count.