Desktop Mini Player Window

Desktop mini-player window

The mini player is not a second application. It is one more window in the same application composition, whose existence is a boolean and whose contents read the same state holder the main window reads:

// adapted
application {
    Window(onCloseRequest = { isVisible = false }, visible = isVisible, …) { App() }
    if (MiniPlayerManager.isOpen) {
        MiniPlayerWindow(
            stateHolder = playerStateHolder,
            MiniPlayerManager.isOpen = false },
        )
    }
}

That boolean lives outside the composition so anything can flip it — the tray menu, an in-app button reached through a platform hook, the window's own close button — with no window handle and no message passing:

// as written
object MiniPlayerManager {
    var isOpen by mutableStateOf(false)
}

Traps

Pass the state holder in; never build a second graph. The temptation with a second window is a second dependency scope and a second view model "for the small one" — then both windows must be told about each other, and in a media app a second graph means a second engine handle. Handing the existing instance down means both collect the same flows and cannot disagree.

Closing the main window must not end the application. Route its close request to visible = false and keep exactly one exit path — typically a tray "quit" item that releases the player first. Otherwise the small window, the whole point of the feature, is destroyed by the gesture users perform to get to it.

If the main window falls back to a decorated, opaque window anywhere, the second one needs the same fallback. Transparent undecorated windows fail to render under some drivers, notably in virtual machines, and the process stays healthy while nothing appears — the failure mode covered by compose-desktop-runtime-hardening. A main window written undecorated = !isVM, transparent = !isVM beside a second written undecorated = true, transparent = true can come up invisible on exactly the machines where the first one was fixed.

Undecorated means you own the drag — and a local drag delta is measured in a moving frame. The handle travels with the window, so each frame's delta is relative to a position that already changed; accumulating it drifts and jitters. Anchor to the OS pointer position captured at drag start, together with the window position, and compute from absolutes every frame:

// adapted — the gesture's own dragAmount is deliberately ignored
detectDragGestures(
   
        val mouse = MouseInfo.getPointerInfo().location
        dragStartMouse = mouse.x to mouse.y
        val pos = windowState.position
        if (pos is WindowPosition.Absolute) dragStartWindow = pos.x.value to pos.y.value
    },
    change, _ ->
        change.consume()
        val startMouse = dragStartMouse
        val startWindow = dragStartWindow
        if (startMouse != null && startWindow != null) {
            val now = MouseInfo.getPointerInfo().location
            windowState.position = WindowPosition(
                (startWindow.first + (now.x - startMouse.first)).dp,
                (startWindow.second + (now.y - startMouse.second)).dp,
            )
        }
    },
    dragStartMouse = null; dragStartWindow = null },
)

Keep the drag handle off the resize edges. A handle spanning the full top edge covers both top resize corners, so the window cannot be resized from them at all; give it the middle portion of the top edge and a fixed height. With no title bar, also set a move cursor on it — that is the only signal the strip is draggable.

The minimum size belongs on the native window, and it is in device pixels. Window state sizes are density-independent; the toolkit's minimum is not, so multiply by the display's scale transform or the floor is off by that factor. Set it from inside the window's content, once it exists:

// adapted
LaunchedEffect(Unit) {
    (window as? java.awt.Window)?.minimumSize = Dimension(
        (minWidth * window.graphicsConfiguration.defaultTransform.scaleX).toInt(),
        (minHeight * window.graphicsConfiguration.defaultTransform.scaleY).toInt(),
    )
}

Enforcing the floor only in Compose lets the native resize go smaller and be corrected on the next frame, which is visible as flicker at the edge of the drag.

Persisted geometry has three separate hazards. Only an absolute position can be written — the sensible default is an alignment (bottom-right of the screen), which has no coordinates, so guard the write on the position's type. Use a sentinel that cannot be a real coordinate for "never saved", because zero is a legitimate position; a not-a-number float works. And clamp the restored size up to the current minimum on read, since a size stored by an older build can be below it. Persist on drag end or debounce, because an effect keyed on the window position runs on every step of a drag — it writes the store continuously while the window moves, and any log line there goes at that rate.

Choose the control set by measured size; let hover do feedback only. The window resizes down to a strip, so branch the layout on its constraints — an aspect-ratio test for a square or tall window, then width thresholds for compact / medium / full — and drop optional buttons below a width. Hover is right for affordances (a cover that scales a little, a panel that goes opaque) and wrong as the only way to reach a control: it has no keyboard equivalent and no touch equivalent.

Wire transport keys on the window, and consume only what you handle — a frameless window with no menu bar has nowhere else to put them. Match on key-down plus the key, act, return true; every other event returns false, or tab focus and every shortcut you did not think of stop working.

Verifying it

1. grep -rn --include="*.kt" "isOpen" . — every write of the open flag in one listing: the tray items, the shared toggle hook, the window's own close request. Any of them holding a window handle or posting an event instead is the path that will disagree with the tray label.
2. grep -rn --include="*.kt" "exitApplication()" . — one hit, not in the main window's close.
3. grep -rn --include="*.kt" -A 3 "alwaysOnTop\|undecorated\|transparent =\|MouseInfo.getPointerInfo\|minimumSize" . — transparent = puts both window declarations in the listing; -A 3 carries the minimum body to the scale multiply. Side by side: the fallback on both; the pointer read in the drag body as well as at start (only at start means local deltas); the minimum scaled by display transform.
4. grep -rn --include="*.kt" -A 3 "LaunchedEffect(windowState" . — anything writing to disk or logging inside an effect keyed on position runs per drag step.
5. By hand: drag slowly across the screen and check it tracks the cursor exactly with no drift; resize from all four corners; shrink to the floor and confirm no flicker; close the main window and confirm the mini player and the audio survive; relaunch and confirm it reopens where and how you left it; delete the stored preferences and confirm the first-run default position is used.