PreferenceFragmentCompat has padding on PreferenceCategory that I can't get rid of

Actually there is a better hack to fix this, also with resource overriding:

Create res/values-sw360dp-v13/values-preference.xml:

<?xml version="1.0" encoding="utf-8"?>

<resources xmlns:tools="http://schemas.android.com/tools">
    <bool name="config_materialPreferenceIconSpaceReserved" tools:ignore="MissingDefaultResource,PrivateResource">false</bool>
    <dimen name="preference_category_padding_start" tools:ignore="MissingDefaultResource,PrivateResource">0dp</dimen>
</resources>

The <bool> fixes the default value of iconSpacePreserved for all Preference; The <dimen> fixes the PreferenceCategory.

EDIT: If you are using androidx.preference:preference:1.1.0-alpha01 or above, you won't need the preference_category_padding_start fix and config_materialPreferenceIconSpaceReserved only will be enough.

I solved by a workaround with styles. It works with PreferenceCategory.

For Preference just use: app:iconSpaceReserved="false".

[styles.xml]

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <item name="preferenceTheme">@style/AppPreferenceTheme</item>
</style>

<style name="AppPreferenceTheme" parent="@style/PreferenceThemeOverlay.v14.Material">
    <item name="preferenceCategoryStyle">@style/FixForPreferenceCategoryStyle</item>

    <!-- <item name="android:textColor">#ffffffff</item> -->
    <!-- <item name="android:textColorSecondary">#b3ffffff</item> -->
    <!--when the check box is checked-->
    <!--<item name="colorControlNormal">#FF4081</item>-->
    <!--when the check box is unchecked -->
    <!--<item name="colorControlActivated">#81FF40</item>-->
</style>

<style name="FixForPreferenceCategoryStyle" parent="@style/Preference.Category.Material">
    <item name="android:layout">@layout/_preference_category_material</item>
</style>

<color name="_preference_fallback_accent_color">@color/colorAccent</color>

[_preference_category_material.xml] copied from "@layout/preference_category_material".

Replace @dimen/preference_category_padding_start with android:paddingLeft="0dp". Add custom color @color/_preference_fallback_accent_color.

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    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="wrap_content"
    android:layout_marginBottom="8dp"
    android:layout_marginTop="8dp"
    android:layout_marginLeft="?android:attr/listPreferredItemPaddingLeft">

    <LinearLayout
        android:id="@+id/icon_frame"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:gravity="start|center_vertical"
        android:orientation="horizontal">
        <android.support.v7.internal.widget.PreferenceImageView
            android:id="@android:id/icon"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:maxHeight="18dp"
            app:maxWidth="18dp"/>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:paddingLeft="0dp"><!-- SET to 0dp -->
        <!--android:paddingLeft="@dimen/preference_category_padding_start"-->

        <TextView
            android:id="@android:id/title"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:paddingRight="?android:attr/listPreferredItemPaddingRight"
            android:textAlignment="viewStart"
            android:textColor="@color/_preference_fallback_accent_color"/>
        <TextView
            android:id="@android:id/summary"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:ellipsize="end"
            android:singleLine="true"
            android:textColor="?android:attr/textColorSecondary"/>
    </LinearLayout>

</FrameLayout>

If using AndroidX, you can add/remove extra padding by simply adding the following attribute to your Preferences and Preference Categories in XML:

<androidx.preference.PreferenceScreen
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <Preference
        ...
        app:iconSpaceReserved="true/false"
        .../>

</androidx.preference.PreferenceScreen>

true adds extra padding, false removes it.