ResizeObserver

gistfile1.txt

// check if ResizeObserver is supported
if( 'ResizeObserver' in window ){
	
	var figcaption = document.querySelector( 'figcaption > h1.entry-title' );
	var figure = document.querySelector( 'figure > div.post-thumbnail' );
	
	// check if the elements exist
	if( ( typeof( figcaption ) !== 'undefined' && figcaption !== null ) &&
		( typeof( figure ) !== 'undefined' && figure !== null ) ){
	
		var ro = new ResizeObserver( entries => {
			for( let entry of entries ){
				
				// get the height, including padding, of .entry-title
				if( entry.target.tagName === 'H1' ){
					var h1 = entry.target;
					var h1cr = entry.contentRect;
					var h1height = h1cr.height + h1cr.top;
				}
				
				// get the height, including padding, of .post-thumbnail
				if( entry.target.tagName === 'DIV' ){
					var div = entry.target;
					var divcr = entry.contentRect;
					var divheight = divcr.height + divcr.top;
				}
				
				// if .entry-title is taller than .post-thumbnail
				if( h1height > divheight ){
					// set the height of the image to the height of .entry-title
					div.querySelector( 'img' ).style.height = h1height + 'px';
				}
			}
		});

		ro.observe( figcaption );
		ro.observe( figure );
	}
}