Layout: one strip, two masks, one control panel

• The strip: one window, MATCH_PARENT wide with adjustable height. TYPE_APPLICATION_OVERLAY (TYPE_PHONE below API 26), PixelFormat.TRANSLUCENT.
• Line highlight mode: a View inside the strip painted with a GradientDrawable in the theme colour; the View's alpha sets the strength. No custom onDraw.
• Reading mask mode: two separate windows, a solid View pinned to Gravity.TOP and one to Gravity.BOTTOM, with the gap between them as the reading window. When you drag the strip, both masks follow.
• Control panel: another window holding the mode switch and the lock button.

Letting touches through to the app underneath

(The code below is simplified for the article.)

A reading ruler is only useful if you can keep scrolling the app under it.

int flags = WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
          | WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL
          | WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN;
if (locked) flags |= WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE; // locked = touches pass through

• The default is locked (touch pass-through). To move the strip you unlock it from the panel, then drag the strip itself with ACTION_DOWN/MOVE/UP and updateViewLayout on params.y.
• When toggling FLAG_NOT_TOUCHABLE, I remove the window and add it again rather than only updating the flags.
• y is clamped between the status bar and the navigation bar, using WindowInsets (statusBars, navigationBars, systemGestures, displayCutout).

Android 12: the opacity cap on pass-through windows

From Android 12 (API 31), an overlay that lets touches pass through to another app has a maximum opacity. Go above it and the system blocks the touches instead (the "untrusted touches" protection).

float maxAlpha = 1f;
if (Build.VERSION.SDK_INT >= 31) {
    InputManager im = getSystemService(InputManager.class);
    maxAlpha = (im != null) ? im.getMaximumObscuringOpacityForTouch() : 0.8f;
}
params.alpha = maxAlpha;

So the window alpha is pinned to that ceiling, and the visible strength comes from the inner View's alpha and the theme colour. This is also why the mask can never be fully black.

Display cutouts

On phones with a notch, the top mask must not leave a gap above it:

• API 30+: LAYOUT_IN_DISPLAY_CUTOUT_MODE_ALWAYS
• API 28–29: LAYOUT_IN_DISPLAY_CUTOUT_MODE_SHORT_EDGES

On rotation, onConfigurationChanged re-clamps the strip and redraws the masks. Every window also has FLAG_KEEP_SCREEN_ON, so the screen doesn't go dark mid-page.

Staying alive and coming back after process death

• A foreground service with foregroundServiceType="specialUse" (FOREGROUND_SERVICE_SPECIAL_USE) and a low-importance, ongoing notification.
• Besides START_STICKY, the overlay is restored on app launch, on process restart, and by a WorkManager watchdog every 15 minutes.
• Users can revoke the overlay permission at any time, and adding a window after that throws BadTokenException, so Settings.canDrawOverlays() is checked before every addView.

The permission flow

ACTION_MANAGE_OVERLAY_PERMISSION with a package: URI → retry without it → fall back to ACTION_APPLICATION_DETAILS_SETTINGS. The guide screen checks canDrawOverlays() in onResume and returns as soon as it's true.

Wrapping up

Most of the work was in small details that let the overlay sit on top without getting in the way. If you're building an overlay of your own, I hope this saves you some time. The app is here (there's a Windows version too):

• Google Play: https://play.google.com/store/apps/details?id=com.codedac.readfocus
• Website: https://codedac.com/apps/readfocus.html

1. Since Android 10, background apps can't read the clipboard

From Android 10 (API 29), only the app that currently has input focus (or the default keyboard) can read the clipboard. The classic clipboard-manager design, a background service that saves every copy, simply stops working. There are workarounds through accessibility services or log permissions, but given Play policy and user trust I didn't want to go there.

So I changed the promise instead of fighting the platform:

• No "auto-save" claim. I removed that wording from the store listing too.

• Save every time the app legitimately gets focus: when the app opens, when you tap the floating panel, and when you use the home-screen widget. The floating panel is an overlay window that is normally FLAG_NOT_FOCUSABLE; on tap it briefly clears that flag to take focus, reads the clipboard, then gives focus back.

• For saving without switching apps, there's an entry in the text selection menu (ACTION_PROCESS_TEXT). Select text anywhere, pick "Clipboard+", and it lands in the history.

You lose "everything you copy just piles up", but "the moment you open the panel to paste, the last thing you copied is saved too" covers most real use.

2. No sync server

Clipboards are full of passwords, addresses, account numbers and one-time codes. I didn't want any of that passing through my server, and as a one-person studio I didn't want to run one forever either.

So the storage is the user's own Google Drive:

• The Android and Windows apps each connect to the user's Drive and sync through the hidden, app-only appDataFolder. It doesn't show up in the Drive UI, and the app can't see the user's other files. Sync uses the drive.appdata scope; backup files use drive.file, which only covers files the app created.

• On Android I skipped the old Drive Android API: AuthorizationClient just gets an access token, and the app calls Drive REST v3 directly. Fewer dependencies, and the same API shape as the Windows side.

• No server means no hosting cost, and I can honestly tell users "your data only lives in your own Drive".

3. Don't hold data hostage when people switch phones

Backup and restore started as paid features. Keeping someone's data locked when they change phones felt wrong, so backup file export/import and Drive restore are now free. Pro is ad removal, live phone-PC sync, unlimited image clips and the larger panel/widget.

Feedback welcome

• Cases where something doesn't get saved (which app, which Android version)

• Whether the "your own Drive, no server" setup feels trustworthy, or whether the permission screen makes it feel the opposite

Google Play: Clipboard+ for Android
Windows (Microsoft Store): Clipboard+ for Windows
More: codedac.com