Fiction narrative is a river of cause and effect which sweeps the reader and the characters through the novel. What happens in each scene affects what happens through the rest of the novel, and main characters should change as these events affect them.
If the sweet heroine has to kill someone to save her lover’s life, that death should change her, and the person’s death should affect the events of the novel.
If that death scene has no effect on either the heroine or the plot, that is a bubble scene.
If she nearly makes love to another man and doesn’t think about her true love and that event does nothing to change her or the plot, that’s a bubble scene.
If your hero saves a gay friend from a bunch of drunks and it has nothing to do with the plot or the character’s goals, get rid of the bubble scene or connect it with the plot and the character’s development.
Bubble scenes are such an emotional disconnect from the rest of the story that they are failures.