Ctrl+Shift held while hovering inventory slots causes severe UI lag in 26.3

When holding Ctrl+Shift while moving the mouse over inventory slots in any
inventory screen (player inventory, chest, etc.), the UI becomes severely
sluggish. The slot highlight animation lags far behind cursor movement,
and all inventory interactions are slowed down.

Steps to Reproduce:

1. Open any inventory screen (player inventory, chest, etc.)

2. Hold Ctrl+Shift

3. Move the mouse cursor over item slots

Expected Result:
UI responds normally, same as without modifier keys held.

Actual Result:
Slot highlight animation lags far behind cursor movement.
All inventory interactions while Ctrl+Shift is held are noticeably slow.

Notes:

• Ctrl+Shift+Right Click is a vanilla feature that bulk-moves all items
  of the same type between a container and player inventory.

• This behavior was normal in versions prior to 26.3.

• Bug is 100% reproducible on every attempt.

Related: MC-307067 (similar Shift key lag reported in 26.1,
different trigger conditions and non-reproducible by original reporter)