WinUIBackend: Kinda fix broken Image alpha blending

Author: stackotter

Sources/WinUIBackend/WinUIBackend.swift

@@ -828,12 +828,23 @@ public final class WinUIBackend: AppBackend {
         let buffer = try! bitmap.pixelBuffer.buffer!
         memcpy(buffer, rgbaData, min(Int(bitmap.pixelBuffer.length), rgbaData.count))
 
-        // Convert RGBA to BGRA in-place.
+        // Convert RGBA to BGRA in-place, and apply janky transparency fix until we
+        // figure out how to fix WinUI image blending (non-black transparent pixels
+        // just don't seem to get blended at all, or at least pixels that are white
+        // enough, haven't tested many colours).
         for i in 0..<(width * height) {
             let offset = i * 4
-            let tmp = buffer[offset]
-            buffer[offset] = buffer[offset + 2]
-            buffer[offset + 2] = tmp
+            if buffer[offset + 3] == 0 {
+                // If transparent, make the pixel black (this is the janky blending fix).
+                buffer[offset] = 0
+                buffer[offset + 1] = 0
+                buffer[offset + 2] = 0
+            } else {
+                // Swap R and B (RGBA to BGRA)
+                let tmp = buffer[offset]
+                buffer[offset] = buffer[offset + 2]
+                buffer[offset + 2] = tmp
+            }
         }
 
         imageView.source = bitmap