Better support to use tree background color with contrast border (fixes #26218)

src/vs/workbench/browser/part.ts

@@ -12,6 +12,7 @@ import { IThemeService, ITheme } from 'vs/platform/theme/common/themeService';
 
 export interface IPartOptions {
 	hasTitle?: boolean;
+	borderWidth?: () => number;
 }
 
 /**
@@ -130,6 +131,10 @@ export class PartLayout {
 		// Content Size: Width (Fill), Height (Variable)
 		const contentSize = new Dimension(width, height - titleSize.height);
 
+		if (this.options && typeof this.options.borderWidth === 'function') {
+			contentSize.width -= this.options.borderWidth(); // adjust for border size
+		}
+
 		sizes.push(titleSize);
 		sizes.push(contentSize);
 

src/vs/workbench/browser/parts/sidebar/sidebarPart.ts

@@ -63,7 +63,7 @@ export class SidebarPart extends CompositePart<Viewlet> {
 			Scope.VIEWLET,
 			SIDE_BAR_TITLE_FOREGROUND,
 			id,
-			{ hasTitle: true }
+			{ hasTitle: true, borderWidth: () => (this.getColor(SIDE_BAR_BORDER) || this.getColor(contrastBorder)) ? 1 : 0 }
 		);
 	}