fix: patch WorkspaceSvg.getGesture to handle keyboard drags

[rachel-fenichel]

Fixes #393

Adds an additional monkeypatch to the workspace. The current Gesture is always null during a keyboard drag, and trying to get a gesture also stops propagation and prevents default on the instigating event.

Without this patch, WorkspaceSvg automatically creates a new Gesture if none was found:

  getGesture(e: PointerEvent): Gesture | null {
    const isStart = e.type === 'pointerdown';

    const gesture = this.currentGesture_;
    if (gesture) {
      if (isStart && gesture.hasStarted()) {
        console.warn('Tried to start the same gesture twice.');
        // That's funny.  We must have missed a mouse up.
        // Cancel it, rather than try to retrieve all of the state we need.
        gesture.cancel();
        return null;
      }
      return gesture;
    }

    // No gesture existed on this workspace, but this looks like the start of a
    // new gesture.
    if (isStart) {
      this.currentGesture_ = new Gesture(e, this);
      return this.currentGesture_;
    }
    // No gesture existed and this event couldn't be the start of a new gesture.
    return null;
  }

The workspace then calls handleWsStart when the event propagates to the workspace, and handleWsStart calls stopPropagation and preventDefault on the event.

Since `handlWsStart is not called in the new approach, directly cancel propagation in the getter.

Additional information

As with anything marked as a monkeypatch, this will need formalization into core.

[maribethb]

typo: should be unpatch not upatch

[rachel-fenichel]

d'oh. fixed.

[cpcallen]

This should have been per-workspace (and patching/unpatching handled carefully) because Mover can handle multiple simultaneous moves on one or more workspaces.