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X
First get this software called "X-Mouse" (Link in description)
Click on latest mirror and download it.
Open it and click RUN after you download it.
Then go through installation process.
I have already installed it so I'll move to the next step.
After installing it, go to your desktop and launch X-Mouse Button Control
Click Layer 2
And give it a name. I named it "Runescape"
Then on Wheel Up change the option to Double Click
Click Apply
and then Close
Go to notification area
Right click the mouse icon
And select Layer 2.
Go to Runescape settings
and toggle mouse buttons.
Mouse buttons (Currently 1)
Drag the item you want to drop in action bar.
Be sure to minimise chat.