What is the difference between an overlay and a notification?

A notification is a request that waits in the shade until dismissed. An overlay draws on top of the current app, requires no action, and disappears on its own.

Why do overlays need a special permission?

Because they can draw over other apps, Android puts SYSTEM_ALERT_WINDOW behind an explicit toggle in Settings, Apps, Special app access, Display over other apps.

Do widgets work while I am using another app?

No. Widgets live on the home screen, so they are only visible between apps.

Android guide

Overlay, notification,
widget — what differs.

August 16, 2026 5 min read

Android gives an app four different ways to put a photo in front of you, and they are not interchangeable. Choosing the wrong one is why so many "photo reminder" apps feel like nagging.

Notifications

A notification is a request. It sits in the shade with a badge, waits to be dismissed, and counts as unfinished business until you deal with it. That is the correct design for a message from a person. It is the wrong design for a photograph of your daughter, because nothing about a memory should require action.

This is what Bubbles In Time was built for: not a feed you scroll and not a notification you open, but your own memories drifting past while you go about your day.

Widgets

A widget lives on the home screen, which means you only see it when you are between apps. Most phone time is spent inside an app, so a home-screen photo widget is invisible for the majority of the day. Widgets are also constrained in how often they can update.

Always-on display

AOD shows content when the screen is otherwise off, and it is deliberately restricted — low refresh, limited colour, tight power budget, and on most devices no arbitrary photo support. It is designed for a clock, not a gallery.

Bubbles In Time uses Android’s display-over-other-apps permission (SYSTEM_ALERT_WINDOW) to float your photos above whatever you’re doing, then get out of the way.

Overlays

An overlay draws on top of whatever app is currently open. It is the only one of the four that can reach you while you are doing something else, which is where most phone time actually goes. It also demands nothing: no badge, no queue, no dismissal. It appears and then it is gone.

That is the whole design difference. A notification interrupts and waits. An overlay appears and leaves.

Why the permission is separate

Because overlays can be misused, Android puts SYSTEM_ALERT_WINDOW behind an explicit settings toggle rather than a standard permission prompt. You grant it in Settings → Apps → Special app access → Display over other apps, deliberately, per app. Any app that wants to draw over your screen has to ask for it in the open, and you can revoke it in the same place.

Bubbles In Time is the Android app this applies to — your own photos drifting over any app, on-device, $2.99 once with no subscription, no ads and no cloud.

Quick answers

Which one reaches me most often? An overlay, because it works inside other apps rather than only on the home screen or lock screen.

Which one is least intrusive? An overlay, if it is transient. Nothing accumulates, nothing needs clearing.

Can I turn it off? Yes — the same settings screen that grants display-over-other-apps revokes it.