Improve transparency on macOS

[madsmtm]

Extension upon #7928, see that for discussion. I've improved a bit on the window background, by setting the alpha channel to follow the window opacity.

When window.opacity < 1, we do two things:

• Set the title bar as transparent (new).
• Disable window border shadows (existing behaviour).

[chrisduerr]

Missing changelog entry.

[sfsam]

I'm seeing the same thing I observed in the discussion of #7928: No borders and a white title bar even though decorations_theme_variant = "Dark" is set. I think this is due to something that was changed in the move to winit 0.3.0. I'm on MacOS Sonoma 14.4.1. FWIW, my window config:

[window]
decorations_theme_variant = "Dark"
decorations = "full"
option_as_alt = "OnlyLeft"
resize_increments = true
dimensions = { columns = 100, lines = 24 }

[nixpulvis]

Set the title bar as transparent (new)

Seems to me this may be undesired. I hate to think we need yet another macOS specific configuration for it though.

[kchibisov]

The end window opacity is not as it should be, it's just generally more opaque compared to what it was before or glitches.

[madsmtm]

I've updated the PR and reverted some of the changes I probably mistakenly thought were for the better, though I'm too unsure of what the desired behaviour is (I don't like transparent windows myself, and don't use Alacritty).

The first two commits should bring Alacritty back in line with what it was pre winit 0.30.

Would be happy if someone else would take over for the rest.

[nixpulvis]

I should be able to finish testing and implementing this PR, though my macOS API knowledge is limited and it may take me a little while to get around to everything. Right now it seems the main things that need changing are with respect to the titlebar and fixing whatever is causing #7969. At first I didn't notice that issue myself, but now transparency is broken for me as well.

I assume the best way to do this would be to make a new PR with the current state of this one and push to that myself as I make progress?

[chrisduerr]

I assume the best way to do this would be to make a new PR with the current state of this one and push to that myself as I make progress?

Probably easiest this way, yes.

[nixpulvis]

So after catching up on this issue, it seems like rust-windowing/winit#3722 fixes the main issue (#7969), so what's left is to figure out what we actually want, if anything.

I'm unsure that having a transparent titlebar is the right default because it can be achieved with window.decorations = "Transparent" (however, I still think that could use with some improvement so the content area doesn't overlap with the titlebar buttons, but that's an existing concern).

I also don't know if it's better to have drop shadows or not with a transparent background. My instinct would be to have them, but given that we don't have them already, I'm fine leaving it as is.

Looking back at #7928, it seems like everything there has been addressed.

Here's a screenshot of an alacritty window (opacity = 0.75) from master with winit pointing to rust-windowing/winit#3722:

and here's one without opacity set:

Yes the transparent window's titlebar is a less dark color, but I don't see the red bleeding into it like it appears to be in the original PR and I believe this behavior looks reasonable.

[madsmtm]

I'll note another option: Use -[NSWindow setAlphaValue:], this also sets the opacity of all the decorations (the border and the title bar) (in this mode, the application should internally render as-if it was opaque)