The bug
When you simply click on input boxes in a certain order, the typing cursor shows for all the boxes. However, what you type will only go in the last box that you clicked.
Note: I get the impression that this happens when you first select a box "below" the next one.
How to reproduce
Go to a place where there is more than one input box (for example in a structure block or in the "add multiplayer server" menu)
Click on all the input boxes in several different orders
Examples
Here are some examples in videos:
Bug additions (Thanks Matolcsy Attila)
The bug is only visual, you can still only edit one box a time
Bug only appears when using mouse clicks, not while using
Shift+TabBug only appears if Server Adress is selected first, not the other way
Code Analysis provided by @unknown found in this comment
Linked issues
is duplicated by 35
MC-147665
The blinking text cursor doesn't disappear when the textbox is deselected in a structure block
Resolved
MC-147759
Jigsaw Block Double Text Line
Resolved
MC-152185
Server ip and server name flashing at the same time
Resolved
MC-154696
Multiple blinking cursors in a gui screen with multiple textfields
Resolved
MC-155973
You can select multiple input boxes at the same time
Resolved
Attachments
Comments 23
Also affects the 'edit server info' screen... presumably every screen with more than one text field in it is affected.
Code analysis using Yarn mappings:
The ParentElement#mouseClicked method iterates over children, returning early if a child handles a mouse click. Since text field widgets handle unfocusing within these clicks (TextFieldWidget#mouseClicked), if a prior child handles a mouse click successfully, then they will never get the chance to check for unfocusing. As a result, clicking a text field widget that's before another text field widget in the children list will not unfocus the latter text field widget.
A potential solution is to separate unfocusing clicking logic in a way that prevents it from being short-circuited.
This also affects jigsaw and command blocks. I updated the ticket to include them.