Three tied-together fixes:
1) captureTab moves from WebContents.capturePage() to CDP
Page.captureScreenshot for every mode (visible / full / region).
Blank-screenshot symptom: after a toolbar-menu selection, the OS
popup teardown left the tab view marked occluded for a few frames
on some Windows setups, so capturePage() snapshotted a
stale/transparent frame at the correct dimensions — no 0x0, no
retry hit. CDP forces a fresh composite regardless of occlusion
state (same path the "Full page" mode was already using) and
returns a base64 PNG directly; PNG dimensions come out of the
IHDR chunk (bytes 16-24). Attach only when nothing else has, and
detach after only if WE attached, so an open DevTools stays
attached.
2) Editor gets a Discard button. Toolbar picks up an "×" glyph next
to Save/Copy that closes the editor tab and drops the working
screenshot. Top-level Escape now falls through the same path
after unwinding an in-flight text placement or crop rectangle. A
new "addon-tab-close" IPC lets an add-on's own tab close itself
(main matches the sender's webContents id against the tab list,
so a page can only close its own tab); window.silentmode.closeTab()
exposes it from addon-tab-preload.js.
3) Manual update controls in Settings > Extensions. New "Check for
updates" button at the top of the Extensions surface calls the
same signed-update polling the boot timer runs; the result is
surfaced inline ("All extensions are up to date" / "N updates
staged; restart Theseus to apply"). A "Pending updates" box
below lists what's in <userData>/addons-updates-staged/ so the
user knows what will be promoted on next restart.
Toolbar-menu popup settle bumped from 120 ms to 250 ms with an
explicit win.focus() in the popup close callback — the previous
window wasn't enough on slower Windows setups. CDP capture no longer
depends on this delay anyway, but the settle still helps any add-on
that does DOM work in its click handler before capture.
Screenshot add-on bumped 0.2.2 → 0.2.3 (Discard button; capture
fixes come from the host, not the add-on).
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