Dirty-diff decorations lost when editor hidden

[pisv]

Bug Description:

When an editor with dirty-diff decorations is hidden and shown again (e.g. when switching editor tabs), the dirty-diff decorations disappear.

Steps to Reproduce:

1. Open a file with diff changes in a text editor. Verify that dirty-diff decorations are shown for the changes.

2. Open another editor. Switch back to the first. Note that there are no dirty-diff decorations now.

Additional Information

The issue is present on the current master. It looks like a regression from #14909. I have checked that the issue is present in 72ca8ed, while the parent commit does not have this issue.

[pisv]

Turns out that setting the editor's model to null removes all decorations owned by the editor. The call stack is

TextModel.removeAllDecorationsWithOwnerId (/eclipse-theia/node_modules/@theia/monaco-editor-core/src/vs/editor/common/model/textModel.ts:1697)
StandaloneEditor._postDetachModelCleanup (/eclipse-theia/node_modules/@theia/monaco-editor-core/src/vs/editor/browser/widget/codeEditor/codeEditorWidget.ts:1885)
StandaloneEditor._postDetachModelCleanup (/eclipse-theia/node_modules/@theia/monaco-editor-core/src/vs/editor/standalone/browser/standaloneCodeEditor.ts:484)
StandaloneEditor.setModel (/eclipse-theia/node_modules/@theia/monaco-editor-core/src/vs/editor/browser/widget/codeEditor/codeEditorWidget.ts:510)
MonacoEditor.handleVisibilityChanged (/eclipse-theia/packages/monaco/src/browser/monaco-editor.ts:273)
EditorPreviewWidget.handleVisiblityChanged (/eclipse-theia/packages/editor/src/browser/editor-widget.ts:64)

[pisv]

The issue is even worse, since an open dirty-diff peek view is also severely affected by just switching editor tabs:

1. Open a file with diff changes in a text editor. Open a dirty-diff peek view for a change. Everything is fine so far:

2. Open another editor. Switch back to the first. Now it looks completely broken. The peek view is no longer seamlessly integrated with the main text editor content but just overlays it:

...and doesn't scroll with it: