unity.md: add drag-and-drop workaround

content/docs/vrchat/unity.md

@@ -82,3 +82,17 @@ Unity may open undocked panel windows (such as color pickers and unitypackage im
 Some workarounds:
 - For KDE, hold the Super key and click anywhere in the window to drag it back onscreen.
 - For KDE, press ALT+F3 -> More Actions -> Move.
+
+### Drag-and-drop does not work
+
+Rather than using Assets -> Import New Asset or Import Package, it's normally possible to drag-and-drop a unitypackage or other asset from a file browser directly into Unity's Project pane, where it will be copied to the Assets folder.
+
+However, if your file browser is running on Wayland, this will show "Failed copying file" errors.
+
+See issue [flathub/com.unity.UnityHub#97](https://github.com/flathub/com.unity.UnityHub/issues/97).
+
+As a workaround, launch your file browser in Xwayland mode. Unity will accept the file as it should.
+
+```
+WAYLAND_DISPLAY="" dolphin
+```