Legend cannot be scrolled to the end if `xref`/`yref` is set to `container`

[attatrol]

If a legend is anchored to the container, the container sizes aren't taken into account when the legend sizes are computed.

In the example codepen legends aren't visible fully and it is not possible to scroll then to the end.

function computeLegendDimensions should take into account the container sizes for calculation of _maxHeight and _maxWidth

[attatrol]

My fix for _maxHeight calculation:

  // - if below/above plot area, give it the maximum potential margin-push value
  // - otherwise, extend the height of the plot area
  legendObj._maxHeight = Math.max(
    (isBelowPlotArea || isAbovePlotArea) ? fullLayout.height / 2 : gs.h,
    30
  );
  // _maxHeight is also limited by the container if it is anchored to it
  if (legendObj.yref === 'container' && legendObj._maxHeight > 30) {
    var yanchor = getYanchor(legendObj);
    var containerAllowedHeight;
    if (yanchor === 'top')
      containerAllowedHeight = legendObj.y * fullLayout.height;
    else if (yanchor === 'bottom')
      containerAllowedHeight = (1 - legendObj.y) * fullLayout.height;
    else // if (yanchor === 'middle')
      containerAllowedHeight = 2 * Math.min(1 - legendObj.y, legendObj.y) * fullLayout.height;
    legendObj._maxHeight = Math.min(containerAllowedHeight, legendObj._maxHeight);
  }