The paragraph will fill th
I don't tink this is a bug: my Android 4 changes the width of p if I move from portrait to landscape and if I double-click to zoom, it fits exactly the screen.
Keep in mind that the viewport for mobiles often shows roughly 1000px (may be 1024) in width, while the screen is smaller; when you say 100% the browser may use the screen width, not the viewport width. This allow you to dblclick to text automatically fit the paragraph to the screen without reflowing. Probably this behaviour is restricted to the P tag and this may be considerated consistent to the semantic of p.