Command Blocks don't have output log when placed

"If a command block doesn't have anything to output then it won't display it in the GUI."
That looks very intended.

Don't think so. Before 38a, the box would appear just without text.

What is the problem then; if the box has nothing to show, why do you really want that box to be here? Where is the bug?

After checking the code source, I find out that this change was made to fix a bug where the previous output was not reset when the last command didn't provide any output (like here MC-74956). The fact that the output box is not displayed if the box is empty is here since 1.7, but because of this bug and the output box default value being "-", the box has no reason to be empty. I can conclude that this change was probably not directly intended so It might indeed be a bug, just wait and see.

Relates to MC-86629

Confirmed.

Without the last output text field the track output button has no context. Additionally when disabling the output "-" is shown as well.

It looks like MC-58634 might have been fixed by having the method net.minecraft.tileentity.CommandBlockBaseLogic.getLastOutput() never return null. This makes the method net.minecraft.client.gui.GuiCommandBlock.updateCmdOutput() set the output to an empty String which is not displayed.

Can't the output text field just be rendered always?

Can confirm in 20w49a.

Can confirm in 21w03a.

Can confirm in 21w05b.