With BetterDisplay you can use a streamed virtual screen to rotate your iPad Sidecar display to achieve portrait Sidecar and scale Sidecar display resolutions beyond the natively available ones.

1. Connect your iPad as secondary Sidecar Display to your Mac (more info: https://support.apple.com/en-us/HT210380). Please make sure of the following:

• You are not mirroring a display to your iPad but Sidecar is configured with the "Use As Separate Display" option in Control Center.
• Both Sideabar and Touchbar are set to "Hide"
• Your iPad is not locked in portrait orientation (to prevent the Sidecar app rotation bug kicking in - the Sidecar app which meant to work only in landscape mode sometimes turns portrait cropping the screen).

2. Under BetterDisplay's Settings > Displays > Overview click Create New Virtual Screen…

3. Under New Virtual Screen Configuration choose Match aspect ratio and associate to a display
4. Under Display to match choose the Sidecar Display
5. Enable Associate the virtual screen with this display and Configure full screen stream to this display
6. Configure Rotation (portrait mode) depending on whether you want landscape or portrait orientation on your Sidecar display. You can decide whether the USB-C (or Lighting) port on your iPad should face down or up.

7. Click Create Virtual Screen

After the operation a streamed image of a newly created virtual screen called Virtual - Sidecar Display should appear on your Sidecar display.

• If you want to change the resolution of the streamed image on the Sidecar display, you can do so by using the resolution slider under Virtual - Sidecar Display in the app menu. Make sure you are not using the Sidecar Display's resolution slider (changing that does not make sense as Sidecar Display is receiving the screen of the display named Virtual - Sidecar Display).
• You can also change the orientation of Virtual - Sidecar Display by clicking on the Rotated Orientation option (this switches between Portrait and Landscape).
• If you changed the orientation or the Virtual - Sidecar Display or in portrait orientation you want to change which direction your iPad's the USB port is facing, you can do so under Stream Virtual Screen menu's rotation controls.

You can discard the streamed virtual screen configuration any time by selecting Discard Virtual Screen under the app menu's Virtual - Sidecar Display > Manage Virtual Screen (or under Settings > Displays > Virtual - Sidecar Display > Discard…)

• If you encounter performance issues, make sure you are using a wired connection (highly recommended) or appropriately fast Wi-Fi connection for virtual screen streaming.
• You might find that the mouse is not rotated or cannot interact with the content on the Sidecar display. The reason for this is when you stream the contents of a virtual screen to the Sidecar, both displays remain in the display layout. This means you can move the mouse to the Sidecar (stream target) screen as well as the virtual screen. If you move it to the Sidecar (stream target) screen, the mouse will not be rotated and will hover over the streamed content only, not interacting with it. You need to move the mouse to the virtual screen (stream source) in order to interact with the content on the virtual screen. The simplest solution is to rearrange the displays in the display layout in a way that the virtual screen will be located next to the primary screen while the Sidecar display should be put farthest or located to a corner in the layout so the mouse does not wander over easily.