No longer able to tab between Edit Key options

repro:
-create a GO with clip and a few keys
-select a key in the Animation window, Curves view
-hit "Enter" (or RMB-Click on a key) to get the Edit Key options (see image)
-hit tab...
bug: The field is not highlighted/selected and ready to be edited.
expected result: tab should switch between the two editable fields, and enter accepts edited value.

Found in Version 5.4.0b15 (Branch: 5.4/release)
not repro in 5.4/animation/dev branch and does not look like it's "animation" related.