When opening a dialog or subapp, such as to edit a page, no feedback is given. The user is not aware that a dialog is now open. Mark up pop-ups with anchors, links or headings that show up as new structural elements to the screen reader. Check also light dialogs as don't announce themselves. The screen reader doesn't know they are opened.

When you tab out of a chooser dialog you end up in the app below. Currently the modality level is only visual but it should also apply to user interaction, in this case tab order. Verify that the tab sequence stays in the current dialog.