fix(datasource/graphene) swap selected layer when activating merge tool to allow place line tool to function. Swap back when tool is unloaded. Also swapping back to the merge tool layer's legacy tool

Author: chrisj

src/datasource/graphene/frontend.ts

@@ -1123,7 +1123,7 @@ class GraphConnection extends SegmentationGraphSourceConnection {
     annotationLayerStates.push(redGroup, blueGroup);
 
     if (layer.tool.value instanceof MergeSegmentsPlaceLineTool) {
-      layer.tool.value = undefined;
+      layer.tool.value = undefined; // unset the line tool if it is active when page is loaded
     }
 
     this.mergeAnnotationState = makeColoredAnnotationState(
@@ -2394,18 +2394,27 @@ class MergeSegmentsTool extends LayerTool<SegmentationUserLayer> {
       return;
     const {
       state: { mergeState },
+      mergeAnnotationState,
     } = graphConnection;
-    if (mergeState === undefined) return;
-    const { merges, autoSubmit } = mergeState;
-
     const lineTool = new MergeSegmentsPlaceLineTool(
       this.layer,
-      graphConnection.mergeAnnotationState,
+      mergeAnnotationState,
     );
+    // Switch selected layer to the layer associated with the tool
+    // to enable to place line tool. Swap back when deactivating.
+    const { selectedLayer } = this.layer.manager.root;
+    const previousSelectedLayer = selectedLayer.layer;
+    const previousSelectedLayerVisible = selectedLayer.visible;
+    const previousTool = tool.value;
+    selectedLayer.layer = this.layer.managedLayer;
+    selectedLayer.visible = true;
     tool.value = lineTool;
     activation.registerDisposer(() => {
-      tool.value = undefined;
+      selectedLayer.layer = previousSelectedLayer;
+      selectedLayer.visible = previousSelectedLayerVisible;
+      tool.value = previousTool;
     });
+    const { merges, autoSubmit } = mergeState;
     const { body, header } =
       makeToolActivationStatusMessageWithHeader(activation);
     header.textContent = "Merge segments";