fix: mascot animation, makes syntax highlighting blink

difflet_test.go

@@ -72,6 +72,52 @@ func TestDiffletStylesOnlyDiffSigns(t *testing.T) {
 	}
 }
 
+func TestDiffletAnimationDoesNotChangeThreadCommentRows(t *testing.T) {
+	if err := applyTheme("dark"); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = applyTheme("dark") })
+
+	app := NewAppWithSettings(
+		&recordingService{}, "owner", "repository", false, 10, 10,
+		AppSettings{Mascot: true, MascotAnimated: true},
+	)
+	app.screen = threadScreen
+	app.loading = false
+	app.details = PRDetails{
+		PullRequest: PullRequest{
+			Owner: "owner", Repository: "repository",
+			RepoWithOwner: "owner/repository", Number: 42,
+			Title: "Stable highlighted comments",
+		},
+		Threads: []ReviewThread{{
+			ID: "thread", Path: "main.go",
+			Comments: []ReviewComment{{
+				ID: "comment", Author: "reviewer",
+				Body: "```go\nfunc main() {\n\tprintln(\"stable\")\n}\n```",
+			}},
+		}},
+	}
+	updated, _ := app.Update(tea.WindowSizeMsg{Width: 100, Height: 30})
+	app = updated.(App)
+	before := strings.Split(app.View(), "\n")
+
+	generation := app.difflet.generation
+	updated, _ = app.Update(diffletTickMsg{generation: generation})
+	app = updated.(App)
+	after := strings.Split(app.View(), "\n")
+
+	if len(before) != len(after) {
+		t.Fatalf("animation changed frame height from %d to %d", len(before), len(after))
+	}
+	for row := diffletHeight; row < len(before); row++ {
+		if before[row] != after[row] {
+			t.Fatalf("animation changed non-mascot row %d:\nbefore %q\nafter  %q",
+				row, before[row], after[row])
+		}
+	}
+}
+
 func TestDiffletEnabledSitsRightOfWrappingHeader(t *testing.T) {
 	app := NewAppWithSettings(
 		&recordingService{}, "owner", "repository", false, 10, 10,