macOS: Always handle background rendering in TerminalWindow

[danielwe]

Taking a crack at fixing #9248. I figured the simplest and most robust solution would be if the macOS app always uses the TerminalWindow background on the Swift side to render the background, while all views including terminal surfaces have clear backgrounds. That way you avoid problems due to partially overlapping views with partial transparency, which is what #9248 is ultimately about. I think the implementation of #8801 would also be simpler once rebased on this.

I'd appreciate reviews of what I have here, so I won't mark this as draft. However, the PR is NOT ready to merge, as the Ventura titlebar tabs changes are not finished. I don't have an environment for testing those at the moment. If someone who does wants to take a crack at finishing this, please go ahead! Otherwise, I'll see what I can figure out within the next couple of days.

To do

• Make necessary changes to Ventura titlebar tabs

[danielwe]

[qwerasd205]

This is not the way. We briefly drew the background color as a part of the apprt view but switched back because doing so causes multiple issues which are unacceptable:

• Breaks alpha-blending, since the blending of text with the default background is done by the system compositor instead of being controlled by us.
• Breaks the behavior of custom shaders that care about the background color.
• Less important, but this also increases the complexity for people using Ghostty code to embed a terminal in another application.

As a side note, if were were to go forward with this solution (which we shouldn't), controlling this behavior by branching on the OS rather than making it runtime configurable by the apprt would not be a good idea.

[danielwe]

I understand, thanks for explaining! Replaced by #9291.