Displaying Post Content on tooltip

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The problem is that your template doesn’t have any quotes around the title attribute at all:

title=<?php echo get_the_content('post_content', $post->ID); ?>

So the browser’s making its best guess and using the first word. So you need to add the quotes:

title="<?php echo get_the_content('post_content', $post->ID); ?>"

Also, if you’re outputting arbitrary content into an attribute you should use the esc_attr() function to escape the content so that any characters like ", <, or > cannot break the HTML:

title="<?php echo esc_attr( get_the_content('post_content', $post->ID ) ); ?>"