fix: detached glass empty after edge-split drop

CLAUDE.md

@@ -31,6 +31,7 @@ bwin borrows real window-construction terms — match their meaning in code, com
 
 - Comment only when it adds something the code doesn't already say.
 - Keep comments to 2 lines max, 100 chars per line. If one genuinely needs more, prefix it with `RATIONAL:`.
+- Wrap code keywords in backticks — API/method names, variable names, identifiers (e.g. `addPane`, `activeDragGlassEl`).
 
 ## Debug sentinel values
 

src/binary-window/glass/drag.js

@@ -24,7 +24,9 @@ export default {
       this.removePane(oldSashId);
 
       const newPaneSash = this.addPane(sash.id, { position: dropArea, id: oldSashId });
-      newPaneSash.domNode.append(activeDragGlassEl);
+      // `addPane` seeds the pane with an empty placeholder glass; replace it with
+      // the dragged glass so the pane holds exactly one (the real) glass.
+      newPaneSash.domNode.replaceChildren(activeDragGlassEl);
     }
   },