Add a test which fails without the change

demo/client.ts

@@ -125,7 +125,7 @@ const xtermjsTheme = {
   foreground: '#F8F8F8',
   background: '#2D2E2C',
   selectionBackground: '#5DA5D533',
-  selectionInactiveBackground: '#444444',
+  selectionInactiveBackground: '#555555AA',
   black: '#1E1E1D',
   brightBlack: '#262625',
   red: '#CE5C5C',

test/playwright/SharedRendererTests.ts

@@ -960,6 +960,32 @@ export function injectSharedRendererTests(ctx: ISharedRendererTestContext): void
     });
   });
 
+  (ctx.skipCanvasExceptions ? test.describe.skip : test.describe)('selectionInactiveBackground', async () => {
+    test.only('should render the the inactive selection when not focused', async () => {
+      const theme: ITheme = {
+        selectionBackground: '#FF000080',
+        selectionInactiveBackground: '#0000FF80'
+      };
+      await ctx.value.page.evaluate(`window.term.options.theme = ${JSON.stringify(theme)};`);
+      await ctx.value.proxy.focus();
+      // Check both the cursor line and another line
+      await ctx.value.proxy.writeln('_');
+      await ctx.value.proxy.write('_');
+      await ctx.value.page.evaluate(`window.term.selectAll()`);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [128, 0, 0, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 2, 1), [128, 0, 0, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 2), [128, 0, 0, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 2, 2), [128, 0, 0, 255]);
+      await ctx.value.page.evaluate(`document.activeElement.blur()`);
+      frameDetails = undefined;
+      // Selection only cell needs to be first to ensure renderer has kicked in
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [0, 0, 128, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 2, 1), [0, 0, 128, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 2), [0, 0, 128, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 2, 2), [0, 0, 128, 255]);
+    });
+  });
+
   test.describe('allowTransparency', async () => {
     test.beforeEach(() => ctx.value.page.evaluate(`term.options.allowTransparency = true`));
 

test/playwright/TestUtils.ts

@@ -400,8 +400,8 @@ export async function pollFor<T>(page: playwright.Page, evalOrFn: string | (() =
       deepStrictEqual(result, val, ([
         `pollFor max duration exceeded.`,
         (`Last comparison: ` +
-          `${typeof result === 'object' ? JSON.stringify(result) : result} !== ` +
-          `${typeof val === 'object' ? JSON.stringify(val) : val}`),
+          `${typeof result === 'object' ? JSON.stringify(result) : result} (actual) !== ` +
+          `${typeof val === 'object' ? JSON.stringify(val) : val} (expected)`),
         `Stack: ${stack}`
       ].join('\n')));
     }