CVE-2026-34774: Electron: Use-after-free in offscreen child window paint callback

Apps that use offscreen rendering and allow child windows via window.open() may be vulnerable to a use-after-free. If the parent offscreen WebContents is destroyed while a child window remains open, subsequent paint frames on the child dereference freed memory, which may lead to a crash or memory corruption.

Apps are only affected if they use offscreen rendering (webPreferences.offscreen: true) and their setWindowOpenHandler permits child windows. Apps that do not use offscreen rendering, or that deny child windows, are not affected.