PlayerList#respawn reuses ServerPlayer without resetting sprinting state or SPEED_MODIFIER_SPRINTING #14413

Expected behavior

When a player respawns or changes dimensions through PlayerList#respawn, player.isSprinting() should be false and Attributes.MOVEMENT_SPEED should not retain LivingEntity.SPEED_MODIFIER_SPRINTING, matching vanilla behavior where PlayerList#respawn constructs a new ServerPlayer instance.

Observed/Actual behavior

Because Paper reuses the existing ServerPlayer instance in PlayerList#respawn (ServerPlayer player = serverPlayer; // Paper - TODO - recreate instance), it manually resets sneaking via player.setShiftKeyDown(false), but never calls player.setSprinting(false) in PlayerList#respawn or ServerPlayer#reset.

In addition, ServerGamePacketListenerImpl#handlePlayerCommand ignores STOP_SPRINTING packets while !this.hasClientLoaded(). If a player dies or enters a portal while sprinting, player.isSprinting() remains true and SPEED_MODIFIER_SPRINTING (+30% movement speed) stays attached to Attributes.MOVEMENT_SPEED in sendAllPlayerInfo after respawn or dimension change.

Steps/models to reproduce

1. Join a Paper server and start sprinting.
2. While sprinting, die and respawn (or enter a Nether/End portal).
3. Inspect player.isSprinting() and player.getAttribute(Attribute.MOVEMENT_SPEED).getModifiers() in PlayerRespawnEvent / PlayerPostRespawnEvent (or in the ClientboundUpdateAttributesPacket sent by PlayerList#sendAllPlayerInfo).
4. Observe that isSprinting() is still true and minecraft:sprinting (0.3 ADD_MULTIPLIED_TOTAL) is still present on the respawned player until a new STOP_SPRINTING packet is received after ServerboundPlayerLoadedPacket.

Plugin and Datapack List

None

Paper version

This server is running Paper version 1.21.11-R0.1-SNAPSHOT (MC: 1.21.11 / 26.3)

Other

In paper-server/patches/sources/net/minecraft/server/players/PlayerList.java.patch, right next to:

player.setShiftKeyDown(false);

calling:

player.setSprinting(false);

clears both the shared sprinting flag (setSharedFlag(3, false)) and SPEED_MODIFIER_SPRINTING on the reused ServerPlayer.