Trace nodes do not nest correctly in the infoview

[eric-wieser]

Prerequisites

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Description

Nested trace nodes are not indented correctly in the widget-based infoview.
They are fine on the command line

Context

This makes set_option trace.Meta.synthInstance true very hard to read the output of.

Zulip thread

Steps to Reproduce

Run

import Lean

#eval Lean.logInfo <|
  .trace { cls := `foo } m!"Foo" #[
    .trace { cls := `bar } m!"Bar 1" #[
      .trace { cls := `baz } m!"Baz" #[
        m!"Log message"
      ]
    ],
    .trace { cls := `bar } m!"Bar 2" #[
      m!"Log message"
    ]
  ]

Expected behavior: Trace nodes are properly nested, as they are on the command line / in #guard_msgs:

[foo] Foo
  [bar] Bar 1
    [baz] Baz
      Log message
  [bar] Bar 2
    Log message

Actual behavior: Infoview shows

Versions

4.15.0-rc1

Additional Information

Impact

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[eric-wieser]

This (mostly) worked in Lean 4.8.0, where the infoview output is:

The bug appeared in 4.9.0.

(This also hopefully exonerates the vscode extension, since the latest extension works ok with old Lean 4.8.0)

[eric-wieser]

I experimented with a patch along the lines of

--- a/src/Lean/Widget/InteractiveDiagnostic.lean
+++ b/src/Lean/Widget/InteractiveDiagnostic.lean
@@ -175,7 +175,9 @@ where
       else
         pure (.strict (← children.mapM (go nCtx ctx)))
     let e := .trace data.cls header data.collapsed nodes
-    return .tag (← pushEmbed e) default
+    -- Trace nodes occupy a full line in the widget view, so ensure the formatter considers them
+    -- as such.
+    return .nest 2 <| .tag (← pushEmbed e) "\n"

which ensures that the TaggedText actually populates the indent correctly; but then the infoview seems to insert extra newlines and indents.