Google Chrome	71.0.3558.0 (Official Build) dev (64-bit)
Firmware Version	Google_Caroline.7820.384.0

Steps to reproduce:
# Download a .docx, .xlsx, or .pptx file (I created these by using "Download as" from Google Docs/Sheets/Slides)
# Open it locally in Chrome OS out of files, opening the Office Editing extension
# Press ctrl + / to open keyboard shortcuts, review the list
Expected: way to activate controls not listed in keyboard shortcuts such download, share, or present buttons
Actual: unable to tab into these controls or access them with another shortcut. The tab order is constrained to the sheet itself.  


Note that there is a bit of a workaround way to get to other controls such as fonts and font sizes: use alt + f to get to Files, press tab to put focus on the menu buttons, and use left/right arrows to navigate to a control.