Don't dim default bg in webgl

Fixes #3970

addons/xterm-addon-webgl/src/RectangleRenderer.ts

@@ -12,6 +12,7 @@ import { IColorSet } from 'browser/Types';
 import { IRenderDimensions } from 'browser/renderer/Types';
 import { RENDER_MODEL_BG_OFFSET, RENDER_MODEL_FG_OFFSET, RENDER_MODEL_INDICIES_PER_CELL } from './RenderModel';
 import { Disposable, toDisposable } from 'common/Lifecycle';
+import { DIM_OPACITY } from 'browser/renderer/Constants';
 
 const enum VertexAttribLocations {
   POSITION = 0,
@@ -208,6 +209,7 @@ export class RectangleRenderer extends Disposable {
 
   private _updateRectangle(vertices: IVertices, offset: number, fg: number, bg: number, startX: number, endX: number, y: number): void {
     let rgba: number | undefined;
+    let isDefault = false;
     if (fg & FgFlags.INVERSE) {
       switch (fg & Attributes.CM_MASK) {
         case Attributes.CM_P16:
@@ -233,6 +235,7 @@ export class RectangleRenderer extends Disposable {
         case Attributes.CM_DEFAULT:
         default:
           rgba = this._colors.background.rgba;
+          isDefault = true;
       }
     }
 
@@ -244,7 +247,7 @@ export class RectangleRenderer extends Disposable {
     const r = ((rgba >> 24) & 0xFF) / 255;
     const g = ((rgba >> 16) & 0xFF) / 255;
     const b = ((rgba >> 8 ) & 0xFF) / 255;
-    const a =  bg & BgFlags.DIM ? 0.5 : 1;
+    const a = (!isDefault && bg & BgFlags.DIM) ? DIM_OPACITY : 1;
 
     this._addRectangle(vertices.attributes, offset, x1, y1, (endX - startX) * this._dimensions.scaledCellWidth, this._dimensions.scaledCellHeight, r, g, b, a);
   }

addons/xterm-addon-webgl/src/atlas/WebglCharAtlas.ts

@@ -241,7 +241,7 @@ export class WebglCharAtlas implements IDisposable {
     if (dim) {
       // Blend here instead of using opacity because transparent colors mess with clipping the
       // glyph's bounding box
-      result = color.blend(this._config.colors.background, color.multiplyOpacity(result, 0.5));
+      result = color.blend(this._config.colors.background, color.multiplyOpacity(result, DIM_OPACITY));
     }
 
     return result;
@@ -283,7 +283,7 @@ export class WebglCharAtlas implements IDisposable {
 
     // Apply dim to the color, opacity is fine to use for the foreground color
     if (dim) {
-      result = color.multiplyOpacity(result, 0.5);
+      result = color.multiplyOpacity(result, DIM_OPACITY);
     }
 
     return result;