Thumbnail Bloom

A KWin desktop effect for Plasma 6. Whenever an inactive window disappears behind another one, Thumbnail Bloom shrinks it into a thumbnail and slides that thumbnail to the nearest free spot on the same screen, so nothing is ever completely hidden. Click a thumbnail to bring its window back to the front, drag it to move the window out of the thumbnail, or right-click it for the window menu.

Windows are never actually moved or resized: they are only painted somewhere else, at a smaller size.

How it works

Only windows of the same screen, virtual desktop and activity are considered. The stack is walked from the top down, so by the time a window is examined, everything covering it has already found its final place.

flowchart TD
    A[Stacking order changed] --> B{Covered by a window above?}
    B -- No --> C[Leave the window alone]
    B -- Yes --> D[Shrink to the initial thumbnail size]
    D --> E{Fits in the nearest free space?}
    E -- Yes --> F[Animate to that spot]
    E -- No --> G[Shrink one step further]
    G --> H{Still above the minimum size?}
    H -- Yes --> E
    H -- No --> I[Place at the minimum size, closest to the window]
    F --> J[Put a transparent click target on the thumbnail]
    I --> J

Loading

Using a thumbnail

Gesture	Result
Left click (on release)	Activates the window and brings it to the front.
Left drag	Activates the window and moves it, starting from the thumbnail.
Right click	Opens the window menu, leaving the window as it is.
Tap	Same as a left click.
Drag with a finger	Same as a left drag.
Press and hold with a finger	Same as a right click.

A drag picks the window up where its thumbnail is, not where the window really sits: the window appears at full size around the thumbnail and follows the pointer (or the finger) from there.

Animations use the system's animation speed, so slowing animations down or turning them off in System Settings applies here too.

Settings

Found under System Settings, Desktop Effects, next to the effect's entry.

Setting	Default	Meaning
Windows kept above others	skipped	Windows with "Keep above others" stay where they are.
Windows on all desktops	skipped	Windows shown on every desktop stay where they are.
Maximized windows	skipped	Maximized and fullscreen windows stay where they are.
Parent windows	skipped	Windows that own a dialog stay where they are.
Child windows	not skipped	Dialogs and other transient windows may become thumbnails.
Initial thumbnail size	50%	Size a thumbnail is tried at first.
Minimum thumbnail size	15%	Smallest size the search may fall back to.
Window icons	shown	Icon drawn to the left of the title of each thumbnail.
Window titles	shown	Title drawn on a plate at the bottom of each thumbnail.
Thumbnail opacity	70%	Opacity of a thumbnail; a hovered one fades to opaque.
3D angle	45°	How far a resting thumbnail is turned away from the viewer.

A skipped window stays out of the effect altogether: it never becomes a thumbnail, and it never pushes another window into one, however much of it the skipped window covers.

Thumbnails at rest are turned towards their original window positions, as if lying at an angle to the screen. The turn flattens out under the pointer, so a hovered thumbnail is always seen head on; setting the angle to zero leaves them flat throughout.

The icon and the title fade out while a thumbnail is hovered, and a thumbnail too small to hold them goes without.

The active window never becomes a thumbnail, and neither do panels, docks, menus or windows being moved or resized.

Building

Requires the KWin development files (kwin-dev on Debian and KDE neon, kwin-devel on Fedora, kwin sources on Arch) matching the running KWin version.

cmake -B build -S . -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_INSTALL_PREFIX=/usr
cmake --build build -j$(nproc)
sudo cmake --install build

Then enable "Thumbnail Bloom" in System Settings, Desktop Effects.

Disabling/Reloading from terminal

To disable the effect from terminal, run:

kwriteconfig6 --file kwinrc --group Plugins --key thumbnailbloomEnabled false
qdbus6 org.kde.KWin /Effects unloadEffect thumbnailbloom
qdbus6 org.kde.KWin /KWin reconfigure

A proper way to reload the effect:

qdbus6 org.kde.KWin /Effects unloadEffect thumbnailbloom
qdbus6 org.kde.KWin /Effects loadEffect thumbnailbloom

If soft reloading doesn't work, restart the compositor so the plugin is reloaded:

kwin_wayland --replace &   # or: kwin_x11 --replace &

Limitations

A compositing effect can only paint; it cannot receive pointer input without grabbing the pointer globally, which an always-on effect must not do. Each thumbnail therefore gets a transparent, focus-less click target of its own, created inside the KWin process and placed exactly on top of the thumbnail.

Because a window keeps its real position while its thumbnail is drawn elsewhere, the area the window used to occupy still reacts to clicks. The covered part of the window was unreachable anyway; the part that was still visible turns into an invisible click target until the window is raised again.