The mouse is considered pressed down when first joining a world

If you clicked to join a world the mouse will be considered to be “pressed down” until you click the mouse again.

Steps to reproduce:

1. Create a new world with commands enabled

2. Run /advancement grant @s everything

3. Leave the world

4. Rejoin the world by either double clicking or clicking the play button

5. Press L (advancements keybind)

6. Move your mouse around

Observe that the advancements screen scrolls around as if you are clicking.

The expected behaviour is that mouse dragging should only happen if the mouse is actively being clicked down.

Code analysis:

This seems to be because during a world load the SDL mouse events are being dropped.
I believe the sequence of events is as follows:
Clicking on open world will first trigger MouseHandler#onButton setting MouseHandler#activeButton to some non-null value. It will then flow into Screen#mouseClicked which eventually calls Minecraft#doWorldLoad.

// Minecraft#doWorldLoad

// ...
while (!this.singleplayerServer.isReady() || this.gui.overlay() != null) {
	// ...
	RenderSystem.pumpEvents(this.sdlEventHandler);
	// ...
}
// ...

RenderSystem#pumpEvents is continuously called here, which calls SDL_EVENTS.SDL_FlushEvents(768, 4871) which will delete any mouse release events (1026) in the queue. So if you released your mouse after Minecraft#doWorldLoad is called it’ll get dropped while the world is loading.

Normally the release mouse event sets MouseHandler#activeButton back to null, but since it gets dropped the active button is never cleared. Then in MouseHandler#handlerAccumulatedMovement the if (this.activeButton != null && ...) check passes and Scree#mouseDragged is called even though there is no mouse button being actively pressed.