I've found an inconsistency between editing script steps with mouse/keyboard and keyboard only in scripts in 16 on a Mac (I haven't tested on Windows). This is a behaviour change in 16, it didn't affect 15.
OS X 10.11.4
iMac Retina 27" Late 2015
3TB Fusion Drive
FileMaker Pro 16 Advanced 22.214.171.124
Behaviour with mouse and keyboard:
Click a script step and then type Space to edit the script step.
Behaviour with keyboard:
Navigate script steps using Up/Down Arrow keys. Type Space to edit the script step. This no longer works for most script steps.
I found it originally with comments, Set Field, Perform Script, Perform Script on Server, Set Error Capture, Allow User Abort, Set Variable and Show Custom Dialog but my testing shows the majority of script steps are affected.
Unaffected script steps:
There is a different behaviour for how the three steps above are selected. Selecting from Up/Down Arrow keys and typing Space opens the 'Specify Calculation' window. Selecting by mouse and typing Space edits the step inline within the script (without opening the 'Specify Calculation' window).
An additional Right Arrow keypress is required to edit those steps that can't be edited.
The space bar will not edit the following script steps regardless of how the script is selected:
Delete All Records
Delete Portal Row
Arrange All Windows
Set Zoom Level
Show/Hide Text Ruler
Set Use System Formats
Allow Formatting Bar
AVPlayer Set Playback State
Enable Touch Keyboard
Install Menu Set
The Right Arrow keypress is required (and sometimes multiple keypresses are required) to edit all of the above steps.