`backdrop-filter` promotes an element to its own compositing layer. A
position:fixed full-screen layer that is created and then hidden was measured to
leave a stale hit-test region behind in Chrome: the page renders perfectly, but
pointer events across the viewport go nowhere.
The report came from a long-lived tab connected to a remote server, where a
connection blip shows and then hides #offlineOverlay. The symptoms were a
terminal that would not scroll and, at the same time, an unrelated
click-to-expand that also stopped responding, while a freshly opened tab was
fine; a read-only console command (getComputedStyle + elementFromPoint, both of
which force a hit-test recomputation) then cured it. Two unrelated features
dying together and one read-only command fixing both points at hit-testing
itself rather than at either feature.
So the `backdrop-filter` moves onto the actually-visible selector and the layer
is never created while hidden. Only the two persistent overlays change:
offline-overlay (toggled with [hidden]) and file-preview-overlay (toggled with
.visible). path-picker and path-preview are created and removed by JS, leave
nothing behind, and are untouched.
⚠️ This is an evidence-based inference, not a fix verified by reproduction:
reproducing it needs a long-lived page that has been through a connection blip,
which I could not manufacture in a controlled environment. The guard test pins
both halves — no such property while hidden, and a real blur while shown — so a
later cleanup cannot quietly delete the effect.
(cherry picked from commit 08442dfee1)
