Fix #5599, long yaml keys trigger explicit mapping (#10874)

Fix #5599, long yaml keys trigger explicit mapping

changelog_unreleased/yaml/10874.md

@@ -0,0 +1,66 @@
+#### Don't switch to explicit mappings needlessly (#10874 by @pdavies)
+
+Previously, Prettier printed long YAML keys using explicit mapping syntax. The intent was to use that syntax to accommodate multiline keys, but it had the unintended effect of using it even for long keys that couldn’t be made multiline – for example, because `--prose-wrap` was set to `preserve` or the key didn’t contain whitespace. This is bad because:
+
+- Doing so has the effect of further increasing the line length.
+- Explicit mappings are obscure and most people have never seen them. It causes confusion to introduce them needlessly.
+
+<!-- prettier-ignore -->
+```yaml
+# Input
+- stage: Process
+  jobs:
+    - template: Process.yml
+      parameters:
+        ${{ if in(parameters.BuildType, 'a') }}:
+          BuildArtifacts:
+            - input: Build.Release.x86
+              output: Processed.Release.x86
+        ${{ if in(parameters.BuildType, 'b                                 ') }}:
+          BuildArtifacts:
+            - input: Build.Release
+              output: Processed.Release
+
+# Prettier stable
+- stage: Process
+  jobs:
+    - template: Process.yml
+      parameters:
+        ${{ if in(parameters.BuildType, 'a') }}:
+          BuildArtifacts:
+            - input: Build.Release.x86
+              output: Processed.Release.x86
+        ? ${{ if in(parameters.BuildType, 'b                                 ') }}
+        : BuildArtifacts:
+            - input: Build.Release
+              output: Processed.Release
+
+# Prettier main
+- stage: Process
+  jobs:
+    - template: Process.yml
+      parameters:
+        ${{ if in(parameters.BuildType, 'a') }}:
+          BuildArtifacts:
+            - input: Build.Release.x86
+              output: Processed.Release.x86
+        ${{ if in(parameters.BuildType, 'b                                 ') }}:
+          BuildArtifacts:
+            - input: Build.Release
+              output: Processed.Release
+
+# Prettier main (with --prose-wrap always)
+- stage: Process
+  jobs:
+    - template: Process.yml
+      parameters:
+        ${{ if in(parameters.BuildType, 'a') }}:
+          BuildArtifacts:
+            - input: Build.Release.x86
+              output: Processed.Release.x86
+        ? ${{ if in(parameters.BuildType, 'b                                 ')
+          }}
+        : BuildArtifacts:
+            - input: Build.Release
+              output: Processed.Release
+```