Christian, I've checked with our product team about the development mode toggle shortcut and discovered that the behavior depends on whether the cursor is focused on the editor.
For PCs, if you have clicked in the LookML editor, then the focus is on one or more lines in the editor. Then control-shift-D will duplicate the line(s) and control-D will delete line(s).
For PCs or Macs, if you click away from the editor, then control-shift-D toggles development mode. For example, you could first click the name of the file on the left or the black or grey color bars on the top of the page, then press control-shift-D.
I've updated the chart above to include this detail, as shown below.
|Duplicate line or section (first click or select lines in editor) ||⌘⇧D ||⌃⇧D |
|Delete current or selected lines (first click or select lines in editor) ||⌘D ||⌃D |
|Toggle development mode on/off (first click away from editor) ||⌃⇧D ||⌃⇧D |