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Try instead:
var parentBlock = Blockly.getMainWorkspace().newBlock('parent_name');
parentBlock.initSvg();
parentBlock.render();
According to all the documentation I can find that should work, but rendering spits out an error in the console:
Uncaught TypeError: a[0].getBoundingRectangle is not a function
getBlocksBoundingBox workspace_svg.ts:1748
getContentMetrics metrics_manager.ts:207
getMetrics metrics_manager.ts:405
resize scrollbar_pair.ts:105
resizeContents workspace_svg.ts:1012
doRenders$$module$build$src$core$render_management render_management.ts:115
triggerQueuedRenders$$module$build$src$core$render_management render_management.ts:76
render block_svg.ts:1554
<anonymous> debugger eval code:1