block-out-from option to show content behind excluded windows (like WDA_EXCLUDEFROMCAPTURE) instead of solid black

[cns1rius]

Feature Description

block-out-from currently replaces the excluded window's texture with a solid black rectangle in the screencast/screen-capture output. This works well for the password-manager privacy use case, but a black block is visually disruptive in recordings where the goal is simply "this window should not appear" — the viewer sees an obvious black rectangle where a window used to be, rather than the content behind it.

I'd like to request an additional mode (or option) where the excluded window is omitted from the captured composition entirely, letting the windows/desktop behind it "show through" — semantically equivalent to Windows' WDA_EXCLUDEFROMCAPTURE flag, where the window simply vanishes from capture and underlying content becomes visible.

Rationale / Use Case

My concrete scenario: I keep AI assistant applications (LLM chat clients, coding agents) running on-screen for personal productivity, but record/share my display for work walkthroughs and tutorials. Currently my options are:

1. block-out-from "screencast" — the AI windows are hidden, but each one renders as an opaque black rectangle in the recording. Viewers see scattered black blocks, which looks broken and unprofessional.
2. Moving to a separate workspace — works, but requires breaking my workflow every time I start or stop a recording, and the scrollable-tiling layout means the workspace transition itself gets captured.
3. Window-level PipeWire capture instead of screen capture — only viable when the recording content is a single window, not for full-display tutorials.

A "show what's behind" mode would make block-out-from usable for the "hide auxiliary applications from recordings" use case without any of these tradeoffs.

[Sempyos]

#3596 and there's another one somewhere that I can't seem to find

[nickjj]

There's also #3272, in which case maybe the opacity portion of the color could control it being transparent or opaque?

One use case for being able to show a different color other than black not discussed yet is layering multiple wallpapers so you can use your monitor as a key light during recordings without showing the key light. For example you could set a white wallpaper and then configure niri to block that layer, so your webcam looks bright on video but folks see your normal wallpaper in the screencast.

[Sempyos]

I just found the other one I meant, but ya
#4379

[antlis]

Same use case here: an AI assistant overlay (layer-shell) that I want visible on my screen but absent from Google Meet shares. The black rectangle from block-out-from is distracting and signals that something is being hidden.

There is an open PR implementing exactly this as a separate hide-from rule for windows and layers, leaving block-out-from unchanged: #3685. It has no review yet and needs a rebase. Omitting the surface instead of drawing black matches what Windows (WDA_EXCLUDEFROMCAPTURE) and macOS (NSWindow.sharingType = .none) do. I have been running a small local patch with that behavior for layer surfaces on 25.08 without problems.