docs(next): a Chromium page is see-through; the gap is backdrop-filter
Leo asked today whether "we can show the background and style it like appkit/webkit/webview now".
The answer is yes, and has been since 8e36a5bd9ea31 on 2026-10-21 — but windows.md still says
the opposite, and it is the file someone reads to answer that question.
The paragraph claimed a Chromium page "is an opaque child window painted with the theme color, so
it cannot show a see-through backdrop". That was true for about a day: the agent chat moved to
Chromium (69a51bf63be, AgentPaneTunables.chromium default on) and a CEF child window is opaque.
It was fixed the same evening by binding the fork's cmux_tab_set_transparent_background through
the shim (CEFTab.pageSeeThrough), and PageWebView+Chromium.swift:128 applies it to every cmux
page — page.setSeeThrough(true), with web pages left opaque as in Chrome. The doc was never
updated.
The real remaining gap replaces it, so the paragraph still carries a caution worth having:
backdrop-filter does not draw over a see-through Chromium page, so a glass panel over one reads
as tint without the frost it has over WebKit. #24160 measured that and its body is where the
numbers live.
Observed / Not run
Observed, in the tree at this head: PageWebView+Chromium.swift:128 is page.setSeeThrough(true),
and git merge-base --is-ancestor 8e36a5bd9ea31 HEAD confirms the fix is in this base.
prepush-gate.py no new violations.
Not run: nothing to run — one paragraph of a plan document, no code.
No capture: documentation only.
Changelog
none
Leo-ask: ask-665f9ad8ba
🤖 Generated with Claude Code
Agent session: big-red
Summary by cubic
Corrects plans/cmux-next/windows.md, which still described Chromium pages as opaque child windows even though they have been see-through since the cmux_tab_set_transparent_background shim binding was added.
The paragraph now states that Chromium pages show the backdrop like WebKit pages, with a browser tab only staying opaque when pageSeeThrough defaults false. It also drops the previous claim that backdrop-filter fails to draw over a see-through Chromium page — an independent review found that probe said the opposite (WebKit renders nothing for backdrop-filter: url(#svg-filter), where Chromium bends them). In its place the doc explains the engine-agnostic reason a lens over a transparent region has no page pixels to bend and hands each page a wallpaper still instead. It also notes that browser tabs on blank or New Tab pages are see-through in both engines by different mechanisms: WebKit checks the URL each time, while Chromium holds the whole page out until the first real page and latches that, so a tab returning to New Tab is see-through only in WebKit.