The pressed Spacebar should show an open hand cursor before click-dragging makes it a closed hand

When holding space (but not dragging), the cursor should show the open hand icon (then switch to the currently-implemented closed hand while mousedown/dragging). Be careful to test in all the cases throughout the editor experience that this meets the desired behavior.

Start by tracing the editor's existing cursor handling for the currently implemented closed hand, then check the Spacebar behavior before and during mousedown/dragging. Done means holding Space shows an open hand before dragging, switches to the closed hand while dragging, and behaves consistently throughout the editor experience.

索引モデルが issue の本文から書いたものです。