Block capture-phase dragstart in the webview so stray native HTML5 drags
(e.g. SVG reorder grips) do not create a stuck translucent GTK surface on
Wayland compositors.
Harden PsyDnD lifecycle when mouseup never reaches the webview: blur,
visibilitychange, pointerup/cancel in capture, and Escape cancel the
floating label without dispatching a drop.
Add -webkit-user-drag:none on sidebar customizer grips.