Showing content behind status and navigation bar

Add the FLAG_LAYOUT_NO_LIMITS flag within the Window. This will work for Android KitKat and above APIs. Example of this would be something like this:

 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
        Window window = getWindow(); 
     window.setFlags(WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS, WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS);
    }

The reason why the FLAG_FULLSCREEN doesn't work is simply because the FULLSCREEN flag is for removing screen decors like status bar(which works for you). However Navigation bar is not a screen decor. The LAYOUT_NO_LIMIT flag allows the window to extend beyond the screen limit, hence Navigation bar also gets covered within this flag.

Also you could simply hide the navigation and status bar by setting the app in immersive mode.

EDIT The API ViewCompat.setOnApplyWindowInsetListener solves issues which the android:fitsSystemWindows=”true” does not and window insets can be applied to a particular view. An example of this.

ViewCompat.setOnApplyWindowInsetsListener(toolbar, (v, insets) -> {
    ViewGroup.MarginLayoutParams params = (ViewGroup.MarginLayoutParams) v.getLayoutParams();
    params.topMargin = insets.getSystemWindowInsetTop();
    return insets.consumeSystemWindowInsets();
});

Standard layouts like FrameLayout , LinearLayout or RelativeLayout will not pass window insets to their children, whereas material layouts will do (e.g. DrawerLayout , CoordinatorLayout ). So we can change our layout to any material one, or we can subclass a standard layout and pass the insets to the children of layout ourselves. We just have to override onApplyWindowInsets method.

@TargetApi(Build.VERSION_CODES.KITKAT)
@Override
public WindowInsets onApplyWindowInsets(WindowInsets insets) {
    int childCount = getChildCount();
    for (int index = 0; index < childCount; ++index)
        getChildAt(index).dispatchApplyWindowInsets(insets); 
        // let children know about WindowInsets

    return insets;
}

Reference Documentation:OnApplyWindowInsetsListener, WindowInsets

I wanted to comment the accepted answer but i can't yet.

Just to warn everyone who'll take this approach

The FLAG_LAYOUT_NO_LIMITS let extend your root view outside of the screen bounds creating other concerns about your layout design. For example, this flag simply breaks form readability by forcing the system to ignore "adjustPan" and "adjustResize" flags

So, even though i still consider Ahbi's answer as the correct one, make sure that nothing breaks outside of the scope of this flag

BETTER SOLUTION:

Actually, i've just found a better way to achieve the same result, without breaking anything

This is my style file

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <item name="android:windowDrawsSystemBarBackgrounds">true</item>
    <item name="android:statusBarColor">@android:color/transparent</item>
    <item name="android:navigationBarColor">@android:color/transparent</item>

</style>

And these are the flags that i set programmatically

w.clearFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION);
w.getDecorView().setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION);

Setting the flag LAYOUT_HIDE_NAVIGATION (in combination with the style items) did the trick and my form still reacts to the adjustPan flag