Per-frame decoration cost: full-screen backdrop-filter flicker in QtWebEngine, standing will-change layers

[jdmanring]

Confirmation dialogs (clear-all etc.) flickered wildly on mouse move in QtWebEngine: their overlays used full-viewport backdrop-filter blur(4px), which forces the compositor to re-rasterize the entire backdrop every invalidated frame.

Sweep of the same defect class across style.css:

• #styled-confirm-overlay / #cookbook-gguf-delete-overlay / #styled-prompt-overlay: blur replaced with flat rgba(0,0,0,0.6) dim (the reported flicker)
• .doc-editor-pane / .notes-pane / .note-fullscreen-overlay: will-change hints that outlive their 200-300ms entry animations, pinning full-pane compositor textures (VRAM the local model needs)
• Six graphics-editor/gallery blurs converted to flat translucency (three were behind 94-96% opaque backgrounds — invisible effect, full per-frame cost; .ge-drop-overlay-msg blurred during drag, the worst case)
• theme-frosted keeps its blur: the effect is the opt-in theme and it already suppresses nested blurs

Guard tests: backdrop-filter allowed only under body.theme-frosted; will-change only mode-scoped; full-screen overlays keep a dim.

Develop commits: 675fe40, 8613e26, 10759b6 (+ test comment-strip fix). Upstream-candidate.

[jdmanring]

Folded into the existing staged branch fix/gpu-compositor-flicker (same theme, June 25) rather than a new branch. Branch now carries: the June interactive-element blur removals + tonight's overlay flicker fix, will-change removals, and the flat-translucency sweep (8d2ea99f..856dfbe4). Two June test verdicts ('faint but intentional — kept') were explicitly reversed with rationale in 856dfbe4.

Root cause of the reported flicker: develop regression — upstream commit 92daf4e5 re-added blur the June branch had removed, and the branch's guard test was branch-only. Develop now carries test_css_no_fullscreen_backdrop_blur.py as a permanent tripwire. Remains open until the upstream PR is filed.

[jdmanring]

Correction to the previous comment after a hostile re-audit: the first consolidation pass was contaminated — a whole-file conflict resolution (git checkout --theirs on style.css) had imported develop's entire 762-line style.css delta into the branch. Rebuilt with hunk-level resolution; the branch diff is now 55 on-theme lines and every guard passes in isolation.

Also split by theme, matching each staged PR's single concern:

• backdrop-filter work → fix/gpu-compositor-flicker (4d11aa39..5d7e3ae3)
• will-change work → fix/css-render-perf (fa8faa54), which already owns the will-change theme; its guard asserts this PR's three removals specifically (a global will-change guard cannot pass on the upstream baseline — other staged branches own the remaining removals; the global sweep guard lives on develop).