Monaco perf improvement: Update the editor focus only when its prop value has changed

[vivek1729]

To update editor's focus state, we check if the passed in prop is true and if the editor is not already focused:

nteract/packages/monaco-editor/src/MonacoEditor.tsx

Lines 382 to 385 in d2088d0

 // Set focus 
 if (editorFocused && !this.editor.hasTextFocus()) { 
   this.editor.focus(); 
 } 

Imagine a scenario where the passed in prop is true but the editor was already focused. We can see that we'd still end up calling the core hastTextFocus api which can lead to low level dom queries and potential layout thrashing.

This can be optimized by checking prevProps, following a similar pattern:

nteract/packages/monaco-editor/src/MonacoEditor.tsx

Lines 317 to 321 in d2088d0

 // Ensures that the source contents of the editor (value) is consistent with the state of the editor 
 // and the value has actually changed. 
 if (prevProps.value !== this.props.value && this.editor.getValue() !== this.props.value) { 
   this.editor.setValue(this.props.value); 
 }