Ensure collapsed mergepane border

packages/nbdime/package.json

@@ -16,7 +16,7 @@
   ],
   "scripts": {
     "build": "tsc --project . && node scripts/copy-files.js",
-    "build:dev": "tsc --project .",
+    "build:dev": "npm run build",
     "clean": "rimraf test/build lib tsconfig.tsbuildinfo",
     "prepublishOnly": "npm run build",
     "test": "jest --coverage",

packages/nbdime/src/styles/common.css

@@ -3,13 +3,15 @@
   /* editor */
   grid-template-columns: 100%;
   grid-auto-rows: minmax(18px, auto);
+  border: var(--codemirror-border);
 }
 
 .nbdime-root .cm-merge-2pane {
   display: grid;
   /* editor | gap | editor */
   grid-template-columns: 47% 6% 47%;
   grid-auto-rows: minmax(18px, auto);
+  border: var(--codemirror-border);
 }
 
 .nbdime-root .cm-merge-2pane > div:first-child {
@@ -25,6 +27,9 @@
   display: grid;
   grid-template-columns: repeat(3, 1fr);
   grid-auto-rows: minmax(18px, auto);
+  gap: var(--jp-border-width);
+  background-color: var(--jp-cell-editor-border-color);
+  border: var(--codemirror-border);
 }
 
 .nbdime-root .cm-merge-left-editor {
@@ -88,8 +93,6 @@
   vertical-align: top;
   width: 100%;
   background-color: var(--jp-cell-editor-background);
-  border: var(--codemirror-border);
-  border-radius: 0px;
 }
 
 .nbdime-root .cm-merge-pane-rightmost {

packages/nbdime/src/styles/merge.css

@@ -20,11 +20,6 @@
   margin-top: 20px;
 }
 
-.jp-Notebook-merge .cm-merge-pane-final {
-  width: 100%;
-  border-top: var(--codemirror-border);
-}
-
 .jp-Notebook-merge .jp-Output-result img {
   max-width: 100%;
 }