decompiler background coloring is fixed width in undefined functions

[mumbel]

Describe the bug
looking at an undefined function in the decompiler where that window is wide enough will show a color change for a fixed width "pane"/"box" for the background. ie in dark theme a fixed width will be purple and then a break and everything to the right is black

Environment (please complete the following information):

• OS: 24.04
• Java Version: 21.0.5
• Ghidra Version: 11.2.1
• Ghidra Origin: official GitHub distro

Additional context
doesn't really matter, just reporting incase that logic could affect/improve other windows