Fixing Pixelated Task Switcher Thumbnails in KDE Plasma

After making the jump from Windows 10 to Linux, I eventually landed on Nobara. It checked all my boxes: it's not Ubuntu-based, it's excellent for gaming, and it features the KDE Plasma desktop.

However, one visual frustration caught my eye right away: the Task Switcher (Alt+Tab). Although the switcher works perfectly fine, the window previews suffer from heavy pixelation, looking as if they are being aggressively downscaled without any filtering or smoothing.

I'm certainly not the only one noticing this. There are various community discussions regarding the poor thumbnail quality in Plasma:

The Problem: Downscaling

Whether KDE uses bilinear or nearest-neighbor sampling isn't the main issue. The problem is that any scaling mode tends to break down when the scale factor drops below 0.5x without proper mipmapping or oversampling.

Unfortunately, there isn't a toggle in the System Settings to fix this. To get high-quality previews, we have to modify some files.

The Solution: A modified style

1. Copy the System Style

We'll use the default thumbnail_grid as a template. Copy the system folder to your local user directory so your changes persist and don't require root access.

Depending on your installation, the source will be at:
/usr/share/kwin-wayland/tabbox/thumbnail_grid/
or
/usr/share/kwin/tabbox/thumbnail_grid/

Copy it to:
~/.local/share/kwin/tabbox/thumbnail_grid_smooth/

2. Edit the QML Logic

We need to tell the Window Manager to render the thumbnails with better texture filtering. Open content/ui/main.qml inside your new folder and locate the KWin.WindowThumbnail block.

Replace that block with the following code:

KWin.WindowThumbnail {
    anchors.fill: parent
    wId: windowId
    
    // --- Workaround for pixelated scaling ---
    layer.enabled: true
    layer.smooth: true
    layer.mipmap: true
    layer.textureSize: Qt.size(width * 2, height * 2)
    // ----------------------------------------
}

Note: Using width * 2 and height * 2 forces the texture to render at a higher resolution before scaling down, resulting in much smoother edges.

3. Update the Manifest

To prevent confusion in the settings menu, we need to give our new style a unique identity. Open metadata.json and update the Id and Name fields:

"Id": "thumbnail_grid_smooth",
"Name": "Thumbnail Grid (Smooth)",

4. Apply the Changes

Once the files are saved:

  1. Open System Settings.
  2. Navigate to Window Management → Task Switcher.
  3. Select Thumbnail Grid (Smooth) from the style list.
  4. Click Apply.

Now, your Alt+Tab previews should look much better!