Item of RecyclerView inside a ConstraintLayout does not occupy the full width of its parent

Question

I'm having a problem using a RecyclerView within a ConstraintLayout, and wanted to confirm that I'm not doing anything wrong before declaring it as a bug. Basically I've defined a RecyclerView inside a ConstraintLayout:

<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v7.widget.RecyclerView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        android:id="@+id/list"/>

</android.support.constraint.ConstraintLayout>

And I've defined its item which is a TextView within a ConstraintLayout (or any other ViewGroup):

<android.support.constraint.ConstraintLayout
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="120dp"
    android:layout_margin="8dp"
    android:background="@color/colorAccent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello" />

</android.support.constraint.ConstraintLayout>

After using a simple adapter to bind some data to this RecyclerView, the output looks like this:

It seems that it gets it right in the next measure cycle, because scrolling down:

And then scrolling back up fixes the first two items whose widths I assume were recalculated:

The ConstraintLayout around the item - which is supposed to take up the whole width of the view - instead seems to wrap_content. If this ConstraintLayout's height is set to wrap_content, the issue goes away. If the TextView is removed, the issue goes away. And finally, if the RecyclerView is not enclosed within a ConstraintLayout, the issue goes away.

Here's the adapter:

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
    private String[] mDataset;

    public static class ViewHolder extends RecyclerView.ViewHolder {
        public ViewGroup vg;
        public ViewHolder(ViewGroup v) {
            super(v);
            vg = v;
        }
    }

    public MyAdapter(String[] myDataset) {
        mDataset = myDataset;
    }

    @Override
    public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,
                                               int viewType) {
        ViewGroup v = (ViewGroup) LayoutInflater.from(parent.getContext())
            .inflate(R.layout.simple_item, parent, false);
        return new ViewHolder(v);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        // Do nothing
    }

    @Override
    public int getItemCount() {
        return mDataset.length;
    }
}

And here's build.gradle:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 26
    buildToolsVersion "27.0.3"
    defaultConfig {
        applicationId "com.ibsplc.icargo.icoandroid.recyclerviewbugtest"
        minSdkVersion 21
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:26'
    compile 'com.android.support:design:26.0.0'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    compile 'com.android.support:recyclerview-v7:26.0.0'
    testCompile 'junit:junit:4.12'
}

Here's a complete example.

Answer 1

As stated in the documentation of ConstraintLayout, you shouldn't use match_parent for its children:

Important: MATCH_PARENT is not recommended for widgets contained in a ConstraintLayout. Similar behavior can be defined by using MATCH_CONSTRAINT with the corresponding left/right or top/bottom constraints being set to "parent"

The proper constraints for your RecyclerView should look like this:

<android.support.v7.widget.RecyclerView
    android:layout_width="0dp"
    android:layout_height="0dp"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    android:id="@+id/list"/>

You should also constrain the TextView in the item's layout by setting at least one vertical and one horizontal constraint:

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Hello"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

Last but not least, you should probably be using the latest version of ConstraintLayout: 'com.android.support.constraint:constraint-layout:1.1.2' instead of 1.0.2.