App 2.6.0: floating button to open the app

The MG4's launcher does not list third-party apps, so the app could
only be opened by reinstalling it. Now a small round MG-red button
floats over the car's screens: tap opens the app, drag moves it (the
position is kept and clamped to the screen), and it hides while the
app itself is open. A switch in the Upload tab turns it off.

The always-on service owns the overlay, so the button is there from
boot. It needs the overlay permission (SYSTEM_ALERT_WINDOW), which the
car build has through the system UID; without it, or if the window
manager refuses, the button stays away and nothing crashes, and the
Upload tab offers the system permission screen.

Tested on an Android 9 AOSP emulator at the head unit's 1920x1080 /
160 dpi with the car APK (installs as UID 1000): button appears over
the home screen, tap opens the app, hidden while open, drag moves it
without opening the app, position survives a force-stop, the switch
works through the UI. FloatingButtonGeometry has 3 unit tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

app/build.gradle

@@ -34,8 +34,8 @@ android {
         applicationId "com.stinnesbeck.nilsmg4"
         minSdk 27
         targetSdk 36
-        versionCode 62
-        versionName "2.5.0"
+        versionCode 63
+        versionName "2.6.0"
 
         // Nothing about the server ships in the APK: its URL and ingest token are entered in
         // the app's Server tab (ServerConfig), and the database is only reachable by the server.

app/src/main/AndroidManifest.xml

@@ -13,6 +13,8 @@
     <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_LOCATION" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_DATA_SYNC" />
+    <!-- The floating button that opens the app; the head unit's launcher does not list it. -->
+    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />
 
     <!-- Wi-Fi auto-join. CHANGE_WIFI_STATE covers enabling the radio and asking the
          supplicant to join a network the unit already has saved; ACCESS_WIFI_STATE covers

app/src/main/java/com/evsuite/abrp/AbrpUploadService.java

@@ -67,6 +67,7 @@ public class AbrpUploadService extends Service {
     private ConsumptionCloudClient consumptionCloudClient;
     private ChargeSessionTracker chargeSessionTracker;
     private TelemetryUploader telemetryUploader;
+    private FloatingButton floatingButton;
     private WifiAutoConnect wifi;
     private ScheduledExecutorService scheduler;
     private SharedPreferences prefs;
@@ -98,6 +99,9 @@ public class AbrpUploadService extends Service {
         chargeSessionTracker = ChargeSessionTracker.get(getApplicationContext());
         wifi = new WifiAutoConnect(getApplicationContext());
         telemetryUploader = new TelemetryUploader(getApplicationContext());
+        // onCreate runs on the main thread, where the overlay window must be added.
+        floatingButton = new FloatingButton(this, prefs);
+        FloatingButton.attach(floatingButton);
 
         scheduler.scheduleWithFixedDelay(
                 this::safeUploadCycle,
@@ -139,6 +143,7 @@ public class AbrpUploadService extends Service {
         prefs.edit().putBoolean("service_running", false).apply();
         if (scheduler != null) scheduler.shutdownNow();
         if (telemetryUploader != null) telemetryUploader.stop();
+        if (floatingButton != null) FloatingButton.detach(floatingButton);
         Log.i(TAG, "Service stopped");
     }
 

app/src/main/java/com/evsuite/abrp/FloatingButton.java

@@ -0,0 +1,224 @@
+package com.evsuite.abrp;
+
+import android.annotation.SuppressLint;
+import android.content.Context;
+import android.content.Intent;
+import android.content.SharedPreferences;
+import android.graphics.PixelFormat;
+import android.graphics.drawable.GradientDrawable;
+import android.os.Handler;
+import android.os.Looper;
+import android.provider.Settings;
+import android.util.DisplayMetrics;
+import android.util.Log;
+import android.view.Gravity;
+import android.view.MotionEvent;
+import android.view.View;
+import android.view.ViewConfiguration;
+import android.view.WindowManager;
+import android.widget.FrameLayout;
+import android.widget.ImageView;
+
+/**
+ * A small round button floating over every app, so the app can be opened on a head unit whose
+ * launcher does not list it. Tap opens the app; drag moves the button, and the position is kept.
+ *
+ * Owned by the always-on {@link AbrpUploadService}, so it is there from boot. Hidden while the
+ * app itself is in front ({@link #setAppInForeground}) and when switched off in the Upload tab.
+ *
+ * Drawing over other apps needs the overlay permission, which the car build holds through the
+ * system UID. Without it — or if the window manager refuses — the button simply stays away:
+ * a third-party app must never crash the head unit (AGENTS.md).
+ *
+ * All methods run on the main thread.
+ */
+final class FloatingButton {
+
+    private static final String TAG = "NilsMG4.Floating";
+    static final String KEY_X = "floating_button_x";
+    static final String KEY_Y = "floating_button_y";
+    private static final int SIZE_DP = 64;
+    private static final int MARGIN_DP = 16;
+    private static final float GLYPH_SCALE = 1.45f;
+
+    private static final Handler MAIN = new Handler(Looper.getMainLooper());
+    private static FloatingButton current;
+    private static boolean appInForeground;
+
+    private final Context context;
+    private final SharedPreferences prefs;
+    private final WindowManager windowManager;
+    private View view;
+    private WindowManager.LayoutParams params;
+    private boolean added;
+
+    FloatingButton(Context context, SharedPreferences prefs) {
+        this.context = context.getApplicationContext();
+        this.prefs = prefs;
+        this.windowManager = (WindowManager) this.context.getSystemService(Context.WINDOW_SERVICE);
+    }
+
+    // ---------- Static hooks for the activity and the settings switch ----------
+
+    static void attach(FloatingButton button) {
+        current = button;
+        button.update();
+    }
+
+    static void detach(FloatingButton button) {
+        if (current == button) current = null;
+        button.removeView();
+    }
+
+    /** MainActivity's onResume/onPause: no button on top of the app itself. */
+    static void setAppInForeground(boolean inForeground) {
+        appInForeground = inForeground;
+        refresh();
+    }
+
+    /** Re-reads the switch and the permission; posted, so callable from anywhere. */
+    static void refresh() {
+        MAIN.post(() -> {
+            if (current != null) current.update();
+        });
+    }
+
+    static boolean canDraw(Context context) {
+        return Settings.canDrawOverlays(context);
+    }
+
+    // ---------- Showing and hiding ----------
+
+    void update() {
+        boolean wanted = UploadSettings.floatingButtonEnabled(prefs) && !appInForeground && canDraw(context);
+        if (wanted && !added) addView();
+        else if (!wanted && added) removeView();
+    }
+
+    private void addView() {
+        try {
+            if (view == null) view = createView();
+            DisplayMetrics metrics = new DisplayMetrics();
+            windowManager.getDefaultDisplay().getRealMetrics(metrics);
+            int size = dp(SIZE_DP);
+            int[] start = FloatingButtonGeometry.defaultPosition(
+                    metrics.widthPixels, metrics.heightPixels, size, dp(MARGIN_DP));
+            params = new WindowManager.LayoutParams(size, size,
+                    WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY,
+                    WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
+                            | WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS,
+                    PixelFormat.TRANSLUCENT);
+            params.gravity = Gravity.TOP | Gravity.START;
+            params.x = FloatingButtonGeometry.clamp(prefs.getInt(KEY_X, start[0]), size, metrics.widthPixels);
+            params.y = FloatingButtonGeometry.clamp(prefs.getInt(KEY_Y, start[1]), size, metrics.heightPixels);
+            windowManager.addView(view, params);
+            added = true;
+        } catch (RuntimeException e) {
+            // BadTokenException, SecurityException, or anything else the platform throws: no button.
+            Log.w(TAG, "Floating button not shown: " + e);
+            added = false;
+        }
+    }
+
+    void removeView() {
+        if (!added) return;
+        try {
+            windowManager.removeView(view);
+        } catch (RuntimeException e) {
+            Log.w(TAG, "Floating button not removed cleanly: " + e);
+        }
+        added = false;
+    }
+
+    private View createView() {
+        FrameLayout frame = new FrameLayout(context);
+        GradientDrawable circle = new GradientDrawable();
+        circle.setShape(GradientDrawable.OVAL);
+        circle.setColor(context.getColor(R.color.ev_accent));
+        circle.setStroke(dp(2), context.getColor(R.color.ev_text_primary));
+        frame.setBackground(circle);
+        frame.setAlpha(0.9f);
+        frame.setElevation(dp(6));
+        frame.setContentDescription(context.getString(R.string.floating_button_description));
+        ImageView glyph = new ImageView(context);
+        glyph.setImageResource(R.drawable.ic_launcher_monochrome);
+        glyph.setScaleType(ImageView.ScaleType.FIT_CENTER);
+        // The launcher glyph keeps the adaptive-icon safe zone (a third of it empty margin);
+        // enlarged here and clipped by the circle, it fills the button instead of a corner of it.
+        glyph.setScaleX(GLYPH_SCALE);
+        glyph.setScaleY(GLYPH_SCALE);
+        frame.addView(glyph, new FrameLayout.LayoutParams(
+                FrameLayout.LayoutParams.MATCH_PARENT, FrameLayout.LayoutParams.MATCH_PARENT));
+        frame.setClipToOutline(true);
+        frame.setOutlineProvider(android.view.ViewOutlineProvider.BACKGROUND);
+        frame.setOnTouchListener(new DragOrTap());
+        return frame;
+    }
+
+    // ---------- Touch: drag moves, tap opens ----------
+
+    private final class DragOrTap implements View.OnTouchListener {
+        private final int touchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
+        private float downRawX, downRawY;
+        private int downX, downY;
+        private boolean dragging;
+
+        @SuppressLint("ClickableViewAccessibility") // a tap opens the app, as a click would
+        @Override
+        public boolean onTouch(View v, MotionEvent event) {
+            switch (event.getActionMasked()) {
+                case MotionEvent.ACTION_DOWN:
+                    downRawX = event.getRawX();
+                    downRawY = event.getRawY();
+                    downX = params.x;
+                    downY = params.y;
+                    dragging = false;
+                    return true;
+                case MotionEvent.ACTION_MOVE: {
+                    float dx = event.getRawX() - downRawX;
+                    float dy = event.getRawY() - downRawY;
+                    if (!dragging && FloatingButtonGeometry.isTap(dx, dy, touchSlop)) return true;
+                    dragging = true;
+                    moveTo(downX + Math.round(dx), downY + Math.round(dy));
+                    return true;
+                }
+                case MotionEvent.ACTION_UP:
+                    if (dragging) {
+                        prefs.edit().putInt(KEY_X, params.x).putInt(KEY_Y, params.y).apply();
+                    } else {
+                        openApp();
+                    }
+                    return true;
+                case MotionEvent.ACTION_CANCEL:
+                    return true;
+                default:
+                    return false;
+            }
+        }
+    }
+
+    private void moveTo(int x, int y) {
+        DisplayMetrics metrics = new DisplayMetrics();
+        windowManager.getDefaultDisplay().getRealMetrics(metrics);
+        params.x = FloatingButtonGeometry.clamp(x, params.width, metrics.widthPixels);
+        params.y = FloatingButtonGeometry.clamp(y, params.height, metrics.heightPixels);
+        try {
+            windowManager.updateViewLayout(view, params);
+        } catch (RuntimeException e) {
+            Log.w(TAG, "Floating button not moved: " + e);
+        }
+    }
+
+    private void openApp() {
+        try {
+            context.startActivity(new Intent(context, MainActivity.class)
+                    .addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_REORDER_TO_FRONT));
+        } catch (RuntimeException e) {
+            Log.w(TAG, "App not opened: " + e);
+        }
+    }
+
+    private int dp(int value) {
+        return Math.round(value * context.getResources().getDisplayMetrics().density);
+    }
+}

app/src/main/java/com/evsuite/abrp/FloatingButtonGeometry.java

@@ -0,0 +1,31 @@
+package com.evsuite.abrp;
+
+/**
+ * Where the floating button sits and whether a touch was a tap. Android-free, unit-tested.
+ * Positions are the button's top-left corner in pixels, as WindowManager takes them.
+ */
+final class FloatingButtonGeometry {
+
+    /** How far down the screen the button starts, as a fraction of the height. */
+    static final float DEFAULT_TOP_FRACTION = 0.4f;
+
+    private FloatingButtonGeometry() { }
+
+    /** Right edge, about 40 % down, inset by [margin] from the edge. */
+    static int[] defaultPosition(int screenWidth, int screenHeight, int size, int margin) {
+        return new int[]{
+                clamp(screenWidth - size - margin, size, screenWidth),
+                clamp(Math.round(screenHeight * DEFAULT_TOP_FRACTION), size, screenHeight)};
+    }
+
+    /** Keeps the whole button on screen, whatever position was saved or dragged to. */
+    static int clamp(int position, int size, int screen) {
+        int max = Math.max(0, screen - size);
+        return Math.max(0, Math.min(position, max));
+    }
+
+    /** A touch that moved no further than the system touch slop is a tap, not a drag. */
+    static boolean isTap(float dx, float dy, int touchSlop) {
+        return dx * dx + dy * dy <= (float) touchSlop * touchSlop;
+    }
+}

app/src/main/java/com/evsuite/abrp/MainActivity.java

@@ -120,7 +120,8 @@ public class MainActivity extends AppCompatActivity {
     private TextView connectionStatusText;
 
     /** Upload page: live telemetry switch and cadence. */
-    private SwitchMaterial liveTelemetrySwitch, boostSwitch;
+    private SwitchMaterial liveTelemetrySwitch, boostSwitch, floatingButtonSwitch;
+    private View overlayPermissionNote;
     private android.widget.Spinner intervalSpinner;
     private TextInputLayout lowSocLayout;
     private TextInputEditText lowSocInput;
@@ -303,6 +304,8 @@ public class MainActivity extends AppCompatActivity {
         super.onResume();
         // Poll state and log only while the screen is up; onPause cancels it.
         uiHandler.post(uiRefresh);
+        FloatingButton.setAppInForeground(true);
+        refreshOverlayPermissionNote();
     }
 
     @Override
@@ -310,6 +313,7 @@ public class MainActivity extends AppCompatActivity {
         super.onPause();
         // Stop polling off-screen: this activity has no reason to spend cycles then.
         uiHandler.removeCallbacks(uiRefresh);
+        FloatingButton.setAppInForeground(false);
     }
 
     // ---------- Update check ----------
@@ -791,6 +795,22 @@ public class MainActivity extends AppCompatActivity {
     // ---------- Upload page ----------
 
     private void bindServicePane() {
+        floatingButtonSwitch = servicePane.findViewById(R.id.floating_button_switch);
+        overlayPermissionNote = servicePane.findViewById(R.id.overlay_permission_note);
+        floatingButtonSwitch.setChecked(UploadSettings.floatingButtonEnabled(prefs));
+        floatingButtonSwitch.setOnCheckedChangeListener((btn, checked) -> {
+            prefs.edit().putBoolean(UploadSettings.KEY_FLOATING_BUTTON, checked).apply();
+            FloatingButton.refresh();
+            refreshOverlayPermissionNote();
+        });
+        servicePane.findViewById(R.id.overlay_permission_button).setOnClickListener(v -> {
+            try {
+                startActivity(new Intent(android.provider.Settings.ACTION_MANAGE_OVERLAY_PERMISSION,
+                        Uri.parse("package:" + getPackageName())));
+            } catch (android.content.ActivityNotFoundException e) {
+                Toast.makeText(this, R.string.overlay_settings_missing, Toast.LENGTH_LONG).show();
+            }
+        });
         liveTelemetrySwitch = servicePane.findViewById(R.id.abrp_upload_switch);
         intervalSpinner = servicePane.findViewById(R.id.interval_spinner);
         boostSwitch = servicePane.findViewById(R.id.boost_switch);
@@ -847,6 +867,16 @@ public class MainActivity extends AppCompatActivity {
         });
     }
 
+    /**
+     * The note under the floating-button switch: only when the button is wanted but this build
+     * may not draw over other apps (the car build may, through the system UID).
+     */
+    private void refreshOverlayPermissionNote() {
+        if (overlayPermissionNote == null) return;
+        boolean missing = UploadSettings.floatingButtonEnabled(prefs) && !FloatingButton.canDraw(this);
+        overlayPermissionNote.setVisibility(missing ? View.VISIBLE : View.GONE);
+    }
+
     /** Clamped to 1-99: 0 would never trigger and 100 would boost permanently. */
     private void saveLowSocThreshold() {
         CharSequence raw = lowSocInput.getText();

app/src/main/java/com/evsuite/abrp/UploadSettings.java

@@ -16,6 +16,13 @@ final class UploadSettings {
     static final String KEY_LOW_SOC_PERCENT = "low_soc_percent";
     static final String KEY_AUTOSTART       = "autostart";
     static final String KEY_ABRP_UPLOAD_ENABLED = "abrp_upload_enabled";
+    /** The floating button that opens the app over the head unit's own screens. */
+    static final String KEY_FLOATING_BUTTON = "floating_button";
+    static final boolean DEFAULT_FLOATING_BUTTON = true;
+
+    static boolean floatingButtonEnabled(SharedPreferences prefs) {
+        return prefs.getBoolean(KEY_FLOATING_BUTTON, DEFAULT_FLOATING_BUTTON);
+    }
 
     /**
      * On by default: an ABRP plan is only useful if it is fed from the moment the drive

app/src/main/res/layout-w1000dp/pane_service.xml

@@ -6,6 +6,12 @@
             <TextView android:id="@+id/status_text" android:layout_width="match_parent" android:layout_height="wrap_content" android:background="@drawable/bg_input_panel" android:padding="16dp" android:layout_marginTop="20dp" android:layout_marginBottom="12dp" android:text="@string/state_stopped" android:textAppearance="@style/Text.EV.Body" />
             <com.google.android.material.switchmaterial.SwitchMaterial android:id="@+id/abrp_upload_switch" android:layout_width="match_parent" android:layout_height="wrap_content" android:minHeight="72dp" android:text="@string/label_abrp_upload_enabled" android:textAppearance="@style/Text.EV.Body" android:paddingTop="8dp" android:paddingBottom="8dp" app:useMaterialThemeColors="true" app:thumbTint="@color/switch_thumb" app:trackTint="@color/switch_track" android:textColor="@color/ev_text_primary" />
             <TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:text="@string/hint_abrp_upload_enabled" android:textAppearance="@style/Text.EV.Secondary" />
+            <com.google.android.material.switchmaterial.SwitchMaterial android:id="@+id/floating_button_switch" android:layout_width="match_parent" android:layout_height="wrap_content" android:minHeight="72dp" android:text="@string/label_floating_button" android:textAppearance="@style/Text.EV.Body" android:paddingTop="8dp" android:paddingBottom="8dp" app:useMaterialThemeColors="true" app:thumbTint="@color/switch_thumb" app:trackTint="@color/switch_track" android:textColor="@color/ev_text_primary" />
+            <TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:text="@string/hint_floating_button" android:textAppearance="@style/Text.EV.Secondary" />
+            <LinearLayout android:id="@+id/overlay_permission_note" android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="vertical" android:layout_marginTop="8dp" android:visibility="gone">
+                <TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:text="@string/overlay_permission_missing" android:textAppearance="@style/Text.EV.Secondary" android:textColor="@color/ev_error" />
+                <com.google.android.material.button.MaterialButton android:id="@+id/overlay_permission_button" style="@style/Widget.EV.Button.Outlined" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_marginTop="8dp" android:text="@string/btn_overlay_permission" />
+            </LinearLayout>
         </LinearLayout>
         <LinearLayout android:layout_width="0dp" android:layout_height="wrap_content" android:layout_weight="1" android:orientation="vertical" android:padding="24dp" android:background="@drawable/bg_glass">
             <TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:text="@string/design_service_timing" android:textAppearance="@style/Text.EV.Eyebrow" />

app/src/main/res/layout/pane_service.xml

@@ -6,6 +6,12 @@
             <TextView android:id="@+id/status_text" android:layout_width="match_parent" android:layout_height="wrap_content" android:background="@drawable/bg_input_panel" android:padding="16dp" android:layout_marginTop="20dp" android:layout_marginBottom="12dp" android:text="@string/state_stopped" android:textAppearance="@style/Text.EV.Body" />
             <com.google.android.material.switchmaterial.SwitchMaterial android:id="@+id/abrp_upload_switch" android:layout_width="match_parent" android:layout_height="wrap_content" android:minHeight="72dp" android:text="@string/label_abrp_upload_enabled" android:textAppearance="@style/Text.EV.Body" android:paddingTop="8dp" android:paddingBottom="8dp" app:useMaterialThemeColors="true" app:thumbTint="@color/switch_thumb" app:trackTint="@color/switch_track" android:textColor="@color/ev_text_primary" />
             <TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:text="@string/hint_abrp_upload_enabled" android:textAppearance="@style/Text.EV.Secondary" />
+            <com.google.android.material.switchmaterial.SwitchMaterial android:id="@+id/floating_button_switch" android:layout_width="match_parent" android:layout_height="wrap_content" android:minHeight="72dp" android:text="@string/label_floating_button" android:textAppearance="@style/Text.EV.Body" android:paddingTop="8dp" android:paddingBottom="8dp" app:useMaterialThemeColors="true" app:thumbTint="@color/switch_thumb" app:trackTint="@color/switch_track" android:textColor="@color/ev_text_primary" />
+            <TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:text="@string/hint_floating_button" android:textAppearance="@style/Text.EV.Secondary" />
+            <LinearLayout android:id="@+id/overlay_permission_note" android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="vertical" android:layout_marginTop="8dp" android:visibility="gone">
+                <TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:text="@string/overlay_permission_missing" android:textAppearance="@style/Text.EV.Secondary" android:textColor="@color/ev_error" />
+                <com.google.android.material.button.MaterialButton android:id="@+id/overlay_permission_button" style="@style/Widget.EV.Button.Outlined" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_marginTop="8dp" android:text="@string/btn_overlay_permission" />
+            </LinearLayout>
         </LinearLayout>
         <Space android:layout_width="1dp" android:layout_height="20dp" />
         <LinearLayout android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="vertical" android:padding="24dp" android:background="@drawable/bg_glass">

app/src/main/res/values/strings.xml

@@ -107,6 +107,12 @@
     <string name="conn_err_timeout">Connection timed out</string>
     <string name="conn_err_http">Server error (HTTP %1$d)</string>
 
+    <string name="label_floating_button">Show floating button</string>
+    <string name="hint_floating_button">A small round button over the car\'s screens that opens this app. Drag it to move it; it hides while the app is open.</string>
+    <string name="overlay_permission_missing">This build may not draw over other apps, so the button cannot appear.</string>
+    <string name="btn_overlay_permission">Allow display over other apps</string>
+    <string name="overlay_settings_missing">The system screen for this permission is not available here</string>
+    <string name="floating_button_description">Open Nils\' MG4</string>
     <string name="label_abrp_upload_enabled">Send live telemetry to the server</string>
     <string name="hint_abrp_upload_enabled">The server stores it and forwards it to ABRP. Turn this off when another app feeds ABRP; consumption and charging tracking keep running.</string>
 

app/src/test/java/com/evsuite/abrp/FloatingButtonGeometryTest.java

@@ -0,0 +1,37 @@
+package com.evsuite.abrp;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+/** The floating button's position and tap rules — pure JVM. */
+public class FloatingButtonGeometryTest {
+
+    @Test
+    public void startsAtTheRightEdgeAboutFortyPercentDown() {
+        // 1920 x 1080 head unit, 64 px button, 16 px margin.
+        assertArrayEquals(new int[]{1840, 432}, FloatingButtonGeometry.defaultPosition(1920, 1080, 64, 16));
+    }
+
+    @Test
+    public void staysFullyOnScreen() {
+        assertEquals(0, FloatingButtonGeometry.clamp(-50, 64, 1920));
+        assertEquals(1856, FloatingButtonGeometry.clamp(5000, 64, 1920));
+        assertEquals(700, FloatingButtonGeometry.clamp(700, 64, 1920));
+        // A saved position from a larger screen still lands inside a smaller one.
+        assertEquals(720 - 64, FloatingButtonGeometry.clamp(1000, 64, 720));
+        // A screen smaller than the button pins it to the corner instead of going negative.
+        assertEquals(0, FloatingButtonGeometry.clamp(10, 64, 40));
+    }
+
+    @Test
+    public void aTouchWithinTheSlopIsATapAnythingFurtherADrag() {
+        assertTrue(FloatingButtonGeometry.isTap(0, 0, 8));
+        assertTrue(FloatingButtonGeometry.isTap(5, -6, 8));    // 7.8 px
+        assertFalse(FloatingButtonGeometry.isTap(6, 6, 8));    // 8.5 px
+        assertFalse(FloatingButtonGeometry.isTap(0, 40, 8));
+    }
+}