Mouse input is partially ignored under high CPU load

Under normal circumstances, the player's camera rotation should strictly follow mouse movement. For example, if I physically move my mouse 5 cm to the right, the in-game camera should rotate 90° to the right.

However, when the CPU is under heavy load, the frame rate decreases, which is especially noticeable on low-performance computers. In previous versions, this behavior was still correct: if I moved my mouse 5 cm to the right, the game would process the delayed input after the lag spike and the camera would still rotate approximately 90° to the right. This behavior was still working correctly in Minecraft version 1.21.11.

Starting from version 26.1.x and all later versions (including the latest snapshots and the current release), there appears to be a mouse input handling issue. If I physically move my mouse 5 cm to the right during a lag spike, the game ignores part of the mouse input. As a result, the camera only rotates about 45° instead of the expected 90°.

I attempted to record a video demonstrating this issue, but either the recording software freezes due to the same performance issue, or the recording fails entirely.

This issue is most noticeable while the game is generating new chunks, when CPU usage is high.

Expected behavior:

Mouse movement should be fully accumulated and processed even when the game is under high CPU load. A lag spike may delay the camera movement, but it should never cause any mouse input to be lost or discarded.

Actual behavior:

A portion of the mouse movement is discarded during low FPS situations, causing the camera rotation to be smaller than the physical mouse movement.