Looking through keyboard shortcuts in the control center, I noticed that Alt+Ctrl+Esc is "kill window." What's the functional difference between that and "closing" the window? I tried it (on an open Firefox window set to a blank page), and nothing obvious happened except that the cursor became a skull.

If you use that, you will lose all data - it's equivalent to 'end task' in windows ... only it actually works - it's a merciless kill, as opposed to a polite request to finish up and close.

Edit:
It is a toggle switch (press again or use to switch off) - while the cursor is a skull, the first window you click will die.

Are you using the "KDE Default for 3 Modifier Keys"?

I always start from "KDE Default for 4 Modifier Keys" & for me this function is Ctrl-Win-Del. It does not toggle, but a secondary (i.e. right for most people) click deactivates the skull.

Ehh...I don't recall changing the default settings, so I should be using that.

Where's that setting? It's not one I've heard of before. I'm quite happy with the ones I've got, though, it's just it would be useful to know where it is in case it gets changed at some point.

The term "Default" in the phrases:
"KDE Default for 3 Modifier Keys"
"KDE Default for 4 Modifier Keys"
may be confusing, especially since KDE appears to default to the 3 Modifier Key "Default".

This leads to the "uniquely unique " idea of a default default.

It's especially bad because I have yet to see a Linux installer that chooses your default for you based on your keyboard.

Anyway, if you didn't change anything, you are probably using the "KDE Default for 3 Modifier Keys" option & can change it as I outlined above.

Sorry again, I thought I had already posted that.

KControl, Alt-F2 kcontrol, or System Configuration (Settings)
>> Regional & Accessibility >> Keyboard Shortcuts --- Shortcut Schemes (tab)
The drop down menu will probably be showing "Current Scheme" but will display the ones I mentioned & several others.
There are 3 sub tabs on "Shortcut Schemes":

• Global Shortcuts
• Shortcut Sequences
• Application Shortcuts

The 1st 2 will show many changes as you play w/ the default setting.

BTW & FWIW, entering "settings:/Accessibility/" in the Konqueror Location bar will take you directly to "Regional & Accessibility"

To add some information to the original question about the difference between "kill" and "close", the difference is the same as between the commands (in command-line) to "kill" and "kill -9". First one, plain "kill", sends a termination signal which means, that the chosen process (in the case of a graphical "point-and-click" method, a window) is signaled to 'please save your work and then exit', which the program may or may not obey - it depends. "Good" programs obey the thing, exit cleanly and be nice. Some programs don't listen to that, or can't listen to that ("jammed"). The second one (infamous "kill -9", same as the skull cursor) sends an immediate kill signal, which the program must obey. That means: if the program would normally "save it's work" and then exit (when given a termination signal), now it isn't given time to "save it's work", but it must exit immediately - perhaps uncleanly.

Read more:
The click-method does the same, but the process to be killed is selected based on which window you click, so you're not asked a process ID or anything like that.

While we're on the subject of "weapons" (i.e. tools to kill with ), don't forget killall & htop.

killall uses the same "kill"/"close" option codes as kill, but its argument(s) are command name(s), not PID's.

htop has recently become a favorite of mine -- I keep instance running in a root xterm &/or tty at all times. Like top it provides a continuously updated (text) screen of your running processes. Unlike top, it is also interactive & allows you to send the various SIGnals, e.g. "kill", "close", etc., to a single process or to a group of them. I find it especially useful when my GUI slows to the speed of cold molasses & the mouse is useless. If I can get to the tty where it is running, then I have a chance to fix things w/o having to power cycle the machine.

In both cases RTM.