When choosing an item or switching between them, a different item is sometimes chosen than the one you clicked on. It seems that if you move your mouse too fast after clicking on an item, the game chooses the item where your POINTER is (about half a second after clicking)and not where you clicked. Minor bug.
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This may be related to: MC-5777, since I noticed both problems appeared when I started using 13w01b. This bug is particularly annoying when trying to separate stacks into groups (such as coal into groups of 8), as the groups will magically restack if you're moving your mouse and clicking too fast.
With the new Inventory Gestures, it needs a small amount of time to register extra clicks, such as double-clicking. This is why the "lag" happens.
It is annoying as you are not used to this delay, causing mess ups.
Yes, you're right about the gestures. However, a new feature which is meant to make inventory management faster shouldn't force us to go slower. Oh well, I guess I'll just... go slowly then.
To fix this bug, Minecraft must check whether the slot selected in MouseDown and MouseUp are the same, or a bug like this happens.
I can agree with you on the part. Also from personal experience, items should be picked up on mousedown, not mouseup.
The item is picked on the MouseUp event, not on the MouseDown, so it's not the matter of Clicking-and-moving the mouse, it's the matter of moving-and-releasing the mouse.