Component used: Fragment Version used: 1.3.0 Devices/Android versions reproduced on: Any

Occurs when we use BottomSheetDialogFragment after updating androidx.fragment:fragment from 1.2.5 to 1.3.0.

Steps to reproduce:

Inherit BottomSheetDialogFragment.
In onCreateView return view inflated from a layout.
In the layout define FrameLayout/FragmentContainer with id container
Add a child fragment into a container with id container Expected: A child fragment is added into our container specified in the layout. Actual result: A child fragment added into the container used for CoordinatorLayout.

Expected view hierarchy (1.2.5):

DecorView
   - LinearLayout
      - FrameLayout
         - FitWindowsFrameLayout(id:action_bar_root)
            - ContentFrameLayout(id:content)
               - FrameLayout(id:container)
                  - CoordinatorLayout(id:coordinator)
                     - View(id:touch_outside)
                     - FrameLayout(id:design_bottom_sheet)
                        - LinearLayout(id:my_bottom_sheet_root)
                           - TextView(id:my_bottom_sheet_text)
                           - FragmentContainerView(id:container)
                              - FrameLayout(id:my_fragment_root)
                                 - TextView(id:my_fragment_text)

Actual view hierarchy (1.3.0):

DecorView
   - LinearLayout
      - FrameLayout
         - FitWindowsFrameLayout(id:action_bar_root)
            - ContentFrameLayout(id:content)
               - FrameLayout(id:container)
                  - CoordinatorLayout(id:coordinator)
                     - View(id:touch_outside)
                     - FrameLayout(id:design_bottom_sheet)
                        - LinearLayout(id:my_bottom_sheet_root)
                           - TextView(id:my_bottom_sheet_text)
                           - FragmentContainerView(id:container)
                  - FrameLayout(id:my_fragment_root)
                     - TextView(id:my_fragment_text)

It seems that it happens because in 1.3.0 a code for finding the right container to use for fragment transaction was changed. A new implementation for DialogFragment is looking for the container starting from the top of hierarchy (DecorView):

@Nullable
public <T extends View> T findViewById(@IdRes int id) {
    return mWindow.findViewById(id);
}

At the same time it's different for Fragment:

@NonNull
FragmentContainer createFragmentContainer() {
    return new FragmentContainer() {
        @Nullable
        public View onFindViewById(int id) {
            if (Fragment.this.mView == null) {
                throw new IllegalStateException("Fragment " + Fragment.this + " does not have a view");
            } else {
                return Fragment.this.mView.findViewById(id);
            }
        }

        public boolean onHasView() {
            return Fragment.this.mView != null;
        }
    };
}

The only workaround we found is to use a different id for our container, anything but container. But this doesn't seem appropriate as we could have a ton of BottomSheets in the project and this kind of id clashing is resolved very well in ordinary fragments.