In equipment tab at the bottom, you can click on +Add Equipment and it will bring up the Equipment, Add Items sidebar.
If you are in the Heward's Handy Haversack and click on +Add items to your Heward's Handy Haversack, it will bring up the description of the Haversack sidebar instead of bringing up the Add Items sidebar.
The work-around is to add items to your equipment and then move it to the Haversack, but it looks like the intention is to be able to add things directly.
In equipment tab at the bottom, you can click on +Add Equipment and it will bring up the Equipment, Add Items sidebar.
If you are in the Heward's Handy Haversack and click on +Add items to your Heward's Handy Haversack, it will bring up the description of the Haversack sidebar instead of bringing up the Add Items sidebar.
The work-around is to add items to your equipment and then move it to the Haversack, but it looks like the intention is to be able to add things directly.