Enforce background fallback to be non-translucent

To avoid screen garbage or wallpaper shining through, the background
we use there for filling the whole needs to

a) Be non-translucent in all cases except if the window is translucent.
b) Don't infer it from the theme as it might be unexpected. Don't define
it in the theme, and use black if it's not defined.

Bug: 28763785
Change-Id: Ifb724b5c5feea3ed70d68e5bd7615fa91185e743
diff --git a/core/java/com/android/internal/policy/DecorView.java b/core/java/com/android/internal/policy/DecorView.java
index f596636..a065219 100644
--- a/core/java/com/android/internal/policy/DecorView.java
+++ b/core/java/com/android/internal/policy/DecorView.java
@@ -45,6 +45,7 @@
 import android.graphics.Rect;
 import android.graphics.Region;
 import android.graphics.Shader;
+import android.graphics.drawable.ColorDrawable;
 import android.graphics.drawable.Drawable;
 import android.os.RemoteException;
 import android.util.DisplayMetrics;
@@ -907,10 +908,12 @@
         if (getBackground() != drawable) {
             setBackgroundDrawable(drawable);
             if (drawable != null) {
-                mResizingBackgroundDrawable = drawable;
+                mResizingBackgroundDrawable = enforceNonTranslucentBackground(drawable,
+                        mWindow.isTranslucent() || mWindow.isShowingWallpaper());
             } else {
                 mResizingBackgroundDrawable = getResizingBackgroundDrawable(
-                        getContext(), 0, mWindow.mBackgroundFallbackResource);
+                        getContext(), 0, mWindow.mBackgroundFallbackResource,
+                        mWindow.isTranslucent() || mWindow.isShowingWallpaper());
             }
             if (mResizingBackgroundDrawable != null) {
                 mResizingBackgroundDrawable.getPadding(mBackgroundPadding);
@@ -1785,7 +1788,8 @@
     private void loadBackgroundDrawablesIfNeeded() {
         if (mResizingBackgroundDrawable == null) {
             mResizingBackgroundDrawable = getResizingBackgroundDrawable(getContext(),
-                    mWindow.mBackgroundResource, mWindow.mBackgroundFallbackResource);
+                    mWindow.mBackgroundResource, mWindow.mBackgroundFallbackResource,
+                    mWindow.isTranslucent() || mWindow.isShowingWallpaper());
             if (mResizingBackgroundDrawable == null) {
                 // We shouldn't really get here as the background fallback should be always
                 // available since it is defaulted by the system.
@@ -1893,21 +1897,41 @@
      * user is resizing the window of an activity in multi-window mode.
      */
     public static Drawable getResizingBackgroundDrawable(Context context, int backgroundRes,
-            int backgroundFallbackRes) {
+            int backgroundFallbackRes, boolean windowTranslucent) {
         if (backgroundRes != 0) {
             final Drawable drawable = context.getDrawable(backgroundRes);
             if (drawable != null) {
-                return drawable;
+                return enforceNonTranslucentBackground(drawable, windowTranslucent);
             }
         }
 
         if (backgroundFallbackRes != 0) {
             final Drawable fallbackDrawable = context.getDrawable(backgroundFallbackRes);
             if (fallbackDrawable != null) {
-                return fallbackDrawable;
+                return enforceNonTranslucentBackground(fallbackDrawable, windowTranslucent);
             }
         }
-        return null;
+        return new ColorDrawable(Color.BLACK);
+    }
+
+    /**
+     * Enforces a drawable to be non-translucent to act as a background if needed, i.e. if the
+     * window is not translucent.
+     */
+    private static Drawable enforceNonTranslucentBackground(Drawable drawable,
+            boolean windowTranslucent) {
+        if (!windowTranslucent && drawable instanceof ColorDrawable) {
+            ColorDrawable colorDrawable = (ColorDrawable) drawable;
+            int color = colorDrawable.getColor();
+            if (Color.alpha(color) != 255) {
+                ColorDrawable copy = (ColorDrawable) colorDrawable.getConstantState().newDrawable()
+                        .mutate();
+                copy.setColor(
+                        Color.argb(255, Color.red(color), Color.green(color), Color.blue(color)));
+                return copy;
+            }
+        }
+        return drawable;
     }
 
     /**
@@ -2037,7 +2061,7 @@
     private void drawResizingShadowIfNeeded(DisplayListCanvas canvas) {
         if (mResizeMode != RESIZE_MODE_DOCKED_DIVIDER || mWindow.mIsFloating
                 || mWindow.isTranslucent()
-                || (mWindow.getAttributes().flags & FLAG_SHOW_WALLPAPER) != 0) {
+                || mWindow.isShowingWallpaper()) {
             return;
         }
         canvas.save();
diff --git a/core/java/com/android/internal/policy/PhoneWindow.java b/core/java/com/android/internal/policy/PhoneWindow.java
index 4f15ece..9ad750d 100644
--- a/core/java/com/android/internal/policy/PhoneWindow.java
+++ b/core/java/com/android/internal/policy/PhoneWindow.java
@@ -509,6 +509,13 @@
     }
 
     /**
+     * @return Whether the window is currently showing the wallpaper.
+     */
+    boolean isShowingWallpaper() {
+        return (getAttributes().flags & FLAG_SHOW_WALLPAPER) != 0;
+    }
+
+    /**
      * Return a LayoutInflater instance that can be used to inflate XML view layout
      * resources for use in this Window.
      *
diff --git a/core/res/res/values/themes.xml b/core/res/res/values/themes.xml
index 998eea5..aecda44 100644
--- a/core/res/res/values/themes.xml
+++ b/core/res/res/values/themes.xml
@@ -174,7 +174,6 @@
 
         <!-- Window attributes -->
         <item name="windowBackground">@drawable/screen_background_selector_dark</item>
-        <item name="windowBackgroundFallback">?attr/colorBackground</item>
         <item name="windowClipToOutline">false</item>
         <item name="windowFrame">@null</item>
         <item name="windowNoTitle">false</item>