The bug
It is not possible to use the Tab (↹) key to focus other GUI elements in the (minecart) command block GUI. Instead only command suggestions are provided.
Pressing Esc does not work either, it completely closes the GUI.
Note: The same applies to beds, but there the only other GUI element is the "Leave Bed" (MC-234824) button and pressing Esc allows you to leave the bed. Therefore not being able to navigate using Tab (↹) there is not as problematic.
Reproduction steps
Place a command block and right click it
Press
Tab(↹) orEscto focus other GUI elements
❌ It does not work
Can confirm. This is also an issue with beds, you cannot use tab to focus the leave bed button.