self.window_size = 3
self.proj = "mean"
self.image_widget.window_funcs = {"t": (getattr(np, self.proj), self.window_size)}

>>> `[range(0, 2), 0, slice(None), slice(None)]`

# use window function is given for this dimension
if self.window_funcs is not None:
    a = array
    for i, dim in enumerate(sorted(numerical_dims)):
        dim_str = curr_scrollable_format[dim]
        dim = dim - i  # since we loose a dimension every iteration
        _indexer = [slice(None)] * (curr_ndim - i)
        _indexer[dim] = indexer[dim + i]

        # if the indexer is an int, this dim has no window func
        if isinstance(_indexer[dim], int):
            a = a[tuple(_indexer)]
        else:
            # if the indices are from `self._get_window_indices`
            func = self.window_funcs[dim_str].func
            window = a[tuple(_indexer)]
            a = func(window, axis=dim)
    return a
else:
    return array[tuple(indexer)]

# curr_ndim=4, i=0
_indexer = [slice(None)] * (curr_ndim - i)
_indexer[dim] = indexer[dim + i]

>>> indexer
Out[4]: [range(0, 2), 0, slice(None, None, None), slice(None, None, None)]
>>> _indexer
Out[5]: 
[range(0, 2),
 slice(None, None, None),
 slice(None, None, None),
 slice(None, None, None)]