KRunner

KRunner is an application built into Plasma 5 to perform functions and run commands quickly, and features a "runner" system to customize functions available for use.

Installation

Install the krunner package.

Usage

To open KRunner in Plasma, you can either right-click the desktop and press "run command", or you can use the default keybindings, Alt+Space or Alt+F2. In some workspaces such as a blank desktop, starting to type will automatically bring up KRunner.

Open KRunner with the Meta key

To set the Meta key as a shortcut to open KRunner, run

$ kwriteconfig5 --file kwinrc --group ModifierOnlyShortcuts --key Meta "org.kde.krunner,/App,,toggleDisplay"

To apply the change to the current session, send a specific D-Bus signal to KWin:

$ qdbus org.kde.KWin /KWin reconfigure

If are not using the D-Bus, then you should restart a KWin instance; run kwin_x11 --replace for X11 and kwin_wayland --replace for Wayland.

Switch active windows

Plasma 5 does not contain default way to specify krunner search only by active window titles. Following approaches are used to work around this issue.

Full list of windows with search by titles

This approach will require xdotool.

Go to System Settings > Workspace > Shortcuts > Custom Shortcuts.
Create new Global shortcut -> Command/URL (by right click)
Tick the checkbox to the right of the name.
In Trigger tab select the desired key combination.
In Action tab type /usr/local/bin/krunner-search-by-windows.sh
Create file /usr/local/bin/krunner-search-by-windows.sh with the following content:
Make file executable and give run permission to all.

Note the space after .

Now you are able to get list of opened windows by specified shortcut and search by this list as you type;

Search by titles without full windows list

This approach is more limited but far less ugly.

Go to System Settings > Workspace > Shortcuts > Custom Shortcuts.
Create new Global shortcut -> D-bus Command (by right click)
Tick the checkbox to the right of the name
In Trigger tab select the desired key combination
In Action tab insert the following: