Slint 1.16's femtovg-wgpu renderer configures the wgpu surface via
get_default_config() and leaves the composite alpha mode at Auto, which
resolves to an opaque surface on KWin/Wayland — a borderless transparent
window then shows a black rectangular backing behind the rounded card.
WGPUConfiguration exposes no surface/alpha hook, so the only fix is a
renderer patch.
Vendor i-slint-renderer-femtovg-1.16.1 with a preferred_alpha_mode() that
picks a supported non-opaque mode (PreMultiplied / PostMultiplied /
Inherit, guarded by the surface's advertised alpha_modes, else Auto) and
wire it into set_surface(). Pulled in via [patch.crates-io]; Slint stays
pinned =1.16.1. The main window is unaffected (it paints opaque).
cx
vicrodh
5fc1beb869