Update Known issues and workaround.

README.md

@@ -38,7 +38,8 @@ Known issues and workaround
 
 Issue | Workaround
 :--- | :---
-In PBR Viewport, texture is fully opaque whereas I set opacity to *n* %. | With your preferred photo editing soft, set opacity **directly** to texture image. Use PNG as exchange format to preserve opacity. Reimport it in SketchUp.
 In PBR Viewport, loading never ends. | Reopen PBR Viewport, accept to "Propagate Materials to Whole Model".
+In PBR Viewport, texture background is white instead of being transparent. | Open PBR Material Editor ("Extensions" > "Physically-Based Rendering" > "Edit Materials..."). Select material. Set "Alpha mode" to "Combined with background".
+In PBR Viewport, texture is fully opaque whereas I set opacity to *n* %. | With your preferred photo editing soft, set opacity **directly** to texture image. Use PNG as exchange format to preserve opacity. Reimport it in SketchUp.
 In PBR Viewport, texture looks weird with perpendicular lines all around. | With your preferred photo editing soft, resize texture image to a [power of two](https://oeis.org/A000079/list) (e.g. *512x512*, *1024x1024*, *2048x2048*, *4096x4096*). Reimport it in SketchUp.
 In PBR Viewport, model disappears. | Press <kbd>F11</kbd> to go fullscreen. If you want to quit fullscreen: press <kbd>F11</kbd> again.