Don't print prelude in terminal mode (#3890)

We can't correctly print simple messages for prelude events when doing progress based display in a terminal, as it would lead to resetting the display of the table rendering.

This does mean that `--show-config` no longer works in the default terminal display mode - but it's not clear it *can* work correctly (at least as currently implemented) since it doesn't cleanly participate in the table rendering.

For cases where `--show-config` is not set (the norm) -nothing would have been printed anyway, so the changes here just avoid resetting the table rendering unnecessarily.

Fixes #3469.

CHANGELOG.md

@@ -2,6 +2,9 @@ CHANGELOG
 =========
 
 ## HEAD (unreleased)
+- Avoid duplicated messages in preview/update progress display.
+  [#3890](https://github.com/pulumi/pulumi/pull/3890)
+
 - Improve CPU utilization in the Python SDK when waiting for resource operations.
   [#3892](https://github.com/pulumi/pulumi/pull/3892)
 

pkg/backend/display/progress.go

@@ -855,9 +855,21 @@ func (display *ProgressDisplay) processNormalEvent(event engine.Event) {
 		// A prelude event can just be printed out directly to the console.
 		// Note: we should probably make sure we don't get any prelude events
 		// once we start hearing about actual resource events.
-
 		payload := event.Payload.(engine.PreludeEventPayload)
-		display.writeSimpleMessage(renderPreludeEvent(payload, display.opts))
+		preludeEventString := renderPreludeEvent(payload, display.opts)
+		if display.isTerminal {
+			display.processNormalEvent(engine.Event{
+				Type: engine.DiagEvent,
+				Payload: engine.DiagEventPayload{
+					Ephemeral: false,
+					Severity:  diag.Info,
+					Color:     cmdutil.GetGlobalColorization(),
+					Message:   preludeEventString,
+				},
+			})
+		} else {
+			display.writeSimpleMessage(preludeEventString)
+		}
 		return
 	case engine.SummaryEvent:
 		// keep track of the summar event so that we can display it after all other