This new alpha build introduces 8 new effects, fixes some crashes and other glitches, fixes compatibility with some old plugins, and adds some debugging options to assist with the WINE effort.
You can read more about 5.2 and what it includes by reading the release notes for the first alpha.
The new effects will be discussed more in the final release notes for 5.2, when the time comes, but here are some previews for now. Each new effect is also available for plugins to use in their Direct2D effect graphs.
Original Image

Linocut

Mosaic

Pointillism

Stained Glass

Ordered Dither

Spherize
(This isn’t actually a very good image to use for showcasing this effect. I’ll have a better example for the final release notes.)

Waves
Here are several examples for this one, which has many different modes you can experiment with:

Halftone

Change Log
Changes since 5.2 Alpha (build 9688):
- New effect: Artistic -> Linocut
- New effect: Artistic -> Mosaic
- New effect: Artistic -> Pointillism
- New effect: Artistic -> Stained Glass
- New effect: Color -> Ordered Dither
- New effect: Distort -> Spherize
- New effect: Distort -> Waves
- New effect: Stylize -> Halftone
- Fixed a crash when trying to use horizontal scrolling in the Unsaved Changes dialog
- Fixed image tabs rendering with a black background
- Fixed compatibility with the legacy plugins “ImPDF Open/Save”, “Chop Up And Save,” and “TR’s Paste Frame”
- Fixed the UI setting “Auto-select nearest visible layer after hiding a layer” (it was doing nothing)
- Optimized performance of render target buffer clearing when Advanced Color is not enabled
- Fixed some RNG in the Frosted Glass effect
- Added extension methods on IFileTypesServiceFactory for plugin developers: LoadFileAsDocument, TryLoadFileAsDocument, LoadFileAsBitmap, and TryLoadFileAsBitmap.
- Fixed the input assignments of the Direct2D DisplacementMapEffect wrapper (it was assigning “input” to “displacement”)
- Removed the /disableCompositionSwapChain command-line parameter
- Added the following command-line parameters to assist with the WINE effort: /disableUIAnimation, /disableComposition, /disableWin11DisplayInformation, and /wine (which is equivalent to the previous 3 together)
- Added ControlInfoPropertyNames.AutoPopulateFileTypes, which when set to true on a FileChooser-typed property will cause it to be auto-populated with all FileTypes (built-ins and plugins) (thanks @toe_head2001!)
Download and Install
This build is available via the built-in updater as long as you have opted-in to pre-release updates. From within Settings -> Updates, enable “Also check for pre-release (beta) versions of paint.net” and then click on the Check Now button. You can also use the links below to download an offline installer or portable ZIP.

You can also ⬇️ download the installer here (for any supported CPU and OS), which is also where you can find downloads for offline installers, portable ZIPs, and deployable MSIs.
Brilliant update Rick, many thanks!!