Display the extension manager in the left sidebar by default. Users will need to acknowledge the disclaimer in the extension manager before using it. (#8050, #8145)
Added blacklist and whitelist support <extension_listings> for the extension manager (#7989)
Automatically link URLs in notebook output text (#8075, #7393)
Added a "Restart Kernel and Run All Cells..." button to the notebook toolbar (#8024)

Added a context menu item for opening a Markdown editor from the Markdown preview (#7942)
Support Node.js 10+ (#8112, #8083)
Added a command to replace the selection in an editor with text (inserting if there is no selection). This can be assigned a keyboard shortcut, as shown below. We also added a command to go through a series of commands and run the first enabled command. (#7908) Here is a keyboard shortcut to insert text in a currently-active notebook editor:
```
{
  command: "notebook:replace-selection",
  selector: ".jp-Notebook",
  keys: ["Ctrl L"],
  args: {text: "lambda x: x"}
}
```
Here is a keyboard shortcut to insert text into an editor of the currently active console, file editor, or notebook:
```
{
  command: "apputils:run-first-enabled",
  selector: "body",
  keys: ["Ctrl L"],
  args: {
    commands: [
      "console:replace-selection",
      "fileeditor:replace-selection",
      "notebook:replace-selection",
    ],
    args: {text: "lambda x: x"}
  }
}
```

NotebookWidgetFactory is now a plugin so it can be overridden (#8066, #7996)
Many improvements to LabIcon: work with all SVG loaders, improve performance, fix issue with menus from extensions (#8125)
Change the header application area to a box panel, which means the header area will display if its children set their minimum height (#8059, #7279)
JupyterLab's custom context menu is now disabled on all descendants of a DOM element with a data-jp-suppress-context-menu attribute (#7877, #7670)

Fix property inspector restoration on reload (#8114)
Increase the timeout for yarn (#8104, #8102)
Fix find and replace with empty strings (#8100, #8098)
Select search text when focusing the search overlay (#8073, #7932)
Fix attaching images with spaces in their names to Markdown cells (#8095)
Fix build errors by distributing the .yarnrc configuration with the Python package (#8045)
Throttle fetch requests in the setting registry's data connector (#7927)
Close the gap between lines in notebook output (#7832, #7760)