Theme editor fields lose focus after typing value in them

### Tested versions

v4.5.1.stable.official [f62fdbde1], v4.7.2.stable.official [ed1daf0bf].
Seen in 4.4+

### System information

Godot v4.5.1.stable - Windows 10 (build 19045) - Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3050 Ti Laptop GPU (NVIDIA; 32.0.15.7703) - AMD Ryzen 5 5600H with Radeon Graphics (12 threads) - 15.34 GiB memory

### Issue description

Not sure if this ever worked properly because I just started working closely with the theme editor but have seen this behaviour before.

When you try to TAB out to the next field in the constant fields panel after typing in a value the next couple of TABs will work properly but then the whole editor UI will lose focus.

If you don't enter anything and just switch between fields with TAB it will work fine.

### Steps to reproduce

1. Create a new theme
2. Add MarginContainer to the theme editor
3. Enable all 4 margins in the constants panel
4. Enter the first margin and try to switch to the next using TAB

https://github.com/user-attachments/assets/a673180a-5d46-4b91-b619-d00b749189b3

### Minimal reproduction project (MRP)

Not project specific.

Start by reproducing the issue in the theme editor using a MarginContainer and the constants panel, comparing TAB navigation after editing with navigation without edits. Trace focus handling from the constants fields and their value-entry updates. Done means TAB continues through the enabled margin fields without the editor UI losing focus.

Written by the indexing model from the issue text.