In tileset editor, attempting to paint terrain over alternative tile instead creates a new alternative tile

[olanti-p]

Tested versions

• Reproducible for v4.3.stable.official [77dcf97]

System information

Godot v4.3.stable - Windows 10.0.22631 - GLES3 (Compatibility) - AMD Radeon RX 6600M (Advanced Micro Devices, Inc.; 30.0.13024.5001) - AMD Ryzen 7 5800H with Radeon Graphics (16 Threads)

Issue description

Attempting to paint terrain over an alternative tile instead results with the terrain editor creating a new alternative tile. Which shouldn't happen, as the alternative tile creation is done via "Select" tab of the tileset editor.

Steps to reproduce

1. Load attached project
2. Open player.tscn
3. Open TileMapLayer node
4. Switch to tileset editing, Paint mode
5. Select property Terrains -> Terrain Set 0 -> Dots Biome
6. Attempt to paint this point:
   
7. Be greeted with this sight:
   

Minimal reproduction project (MRP)

TerrainPainterBug.zip

[olanti-p]

I tracked down the issue to the "create alternative tile" button that's appended to the end of the row.

Here in this pic, it's changed up to use grey background instead of transparent.

For some reason, the button is missing the icon, is sized incorrectly and overlaps nearby squares, intercepting terrain editor input.