grid - only draw border around tabs as needed

src/vs/workbench/browser/parts/editor/media/tabstitlecontrol.css

@@ -37,7 +37,6 @@
 	cursor: pointer;
 	height: 35px;
 	box-sizing: border-box;
-	border: 1px solid transparent;
 	padding-left: 10px;
 }
 

src/vs/workbench/browser/parts/editor/tabsTitleControl.ts

@@ -771,9 +771,11 @@ export class TabsTitleControl extends TitleControl {
 		// Label
 		this.redrawLabel(editor, tabContainer, tabLabelWidget, tabLabel);
 
-		// Borders
-		tabContainer.style.borderLeftColor = (index !== 0) ? (this.getColor(TAB_BORDER) || this.getColor(contrastBorder)) : null;
-		tabContainer.style.borderRightColor = (index === this.group.count - 1) ? (this.getColor(TAB_BORDER) || this.getColor(contrastBorder)) : null;
+		// Borders / Outline
+		const borderLeftColor = (index !== 0) ? (this.getColor(TAB_BORDER) || this.getColor(contrastBorder)) : null;
+		const borderRightColor = (index === this.group.count - 1) ? (this.getColor(TAB_BORDER) || this.getColor(contrastBorder)) : null;
+		tabContainer.style.borderLeft = borderLeftColor ? `1px solid ${borderLeftColor}` : null;
+		tabContainer.style.borderRight = borderRightColor ? `1px solid ${borderRightColor}` : null;
 		tabContainer.style.outlineColor = this.getColor(activeContrastBorder);
 
 		// Settings