Check if the button appears next to all block elements except the image (no toolbar item available).
Change the format of one of the block elements, panel attached to button should hide after that.
Put the selection in the one of the last blocks, click the button to display the panel then start resizing the browser window and observe if the button and panel are properly repositioned.
External changes
Typing
Click Start external typing
Put the selection in the first paragraph (Like all the great things...) and click the button to open the panel (be quick).
Check if the button and the panel are repositioned correctly.
Removing
Click Start external deleting
Put the selection in the first paragraph (Like all the great things...) and click the button to open the panel (be quick).