When typing in the script editor, I expect syntax autocompletion, for example math. pops up the window, I type “r”, and the first function is selected, then I type ctrl-T (tab) and the function is completed in the editor.
Instead, when I type “r”, the cursor moves from the editor, the “r” doesn’t appear in the editor, the correct method is selected in the autocomplete, but pressing tab does not return the cursor or complete the function in the editor.
This bug happens often, but not all the time. Sometimes the behavior is as expected. Maybe a race condition with setting focus? This occurs in a new project and when editing a brand new script.
This is in Roblox Studio Version 0.405.0.348052 and Mac OS 10.14.5 (18F132) 
Here’s a video showing the issue:
This probably started in the last few days, though I’m not exactly sure of the last time I used Script Editor.
Let me know if you need any more info! 
-Paralusuz