This GMEdit plugin allows you to quickly open a script for viewing/editing in a side panel.
It is largely inspired by Peek Definition in Visual Studio and GameMaker Studio 2's own workflow with layered popouts.
The default keybindings are Alt-F1 / Alt-F12; keybinding name is "Peek Aside";
Performing the keybinding inside the side panel will open a new file in it while using the regular "Open declaration" binding (default: F1 / F12) will switch and focus the main panel, allowing you to juggle editors.
You can close the side view using Esc or Ctrl+W. If the side view's file is not currently active in the main panel, the file will be closed.