Fix macOS Metal window transparency

[vcali]

Closes #1500.

Summary

• preserve alpha on the direct macOS Metal path instead of forcing the drawable/window background back to opaque
• let transparent Metal windows use the Cocoa window background tint instead of clearing the Metal drawable with a semi-transparent terminal background
• keep the custom macOS view/layer non-opaque so AppKit/Core Animation can composite the window correctly
• add focused renderer tests for the new transparency decision logic

Root cause

The 0.3.x macOS Metal path could still end up visually opaque even with window.opacity < 1.0 because background updates restored alpha to 1.0, and the renderer was still clearing the Metal drawable directly for transparent windows.

Testing

• cargo test -p rioterm

[raphamorim]

I apologise taking time to reply, will look this soon

[vcali]

I'll fix the conflicts later today

[aekasitt]

Installed via cargo as such

cargo install --git https://github.com/raphamorim/rio.git --rev refs/pull/1501/head

Works !!

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Workstation details: Apple M1 Pro (Tahoe 26.3)

[lazo4]

It worked for me a few days ago, but when I try it now, I get:

Notes: I have an M1 MacBook Air, running macos 26.4

[vcali]

@lazo4 can you get me your config please?

[lazo4]

Sure, I reduced it to the minimal config:

[window]
opacity = 0.5
blur = true

All the other options have been left to their defaults

Important Note

It used to work a few weeks ago when I first tried it, so it’s possible something broke in main.
I also have this issue on my 2019 Intel imac, which is running macos 15.7.4.

[raphamorim]

Should be fixed in #1576 thank you !