By default alt + mouse is used for window specific shortcuts (moving,resizing). But alt + mouse is used by 3D apps like blender, freecad or maya (not necessary default) to navigate viewport. My proposal is to change alt to meta as it stays unused and is somewhat system key so it will be more intuitive to use it - system key for system.
I've roughly checked and gnome default setting is to use meta key to move windows but ubuntu defaults to alt for both unity and gnome not sure why. Fedora uses default meta. Can check more distros if you want. Another app using alt+click is Inkscape to select object below. Blender also uses alt+click for loop selection. It's not a problem to change that shortcut especially that KDE has this setting visible without need to install any tweak. But changing default will reduce googling for first-time users.
Alt+click has a decades-long tradition in KDE and other places. Apps really shouldn't be using this for anything.
Yep, I'm aware of this. But I'm also aware that when more sophisticated selection is involved inside app it's most probably pretty hard to workaround this. Next one: GIMP: alt+click show layer mask This list is short, but can show that if you want to do some creative things you will probably have to change that shortcut inside system settings.
There is a window specific rule to block shortcuts. If an app requires this: use it. For everything else: once there is a cross desktop standard on shortcuts we might consider. But we certainly won't break everybody's habit because of apps not working.
That's why I've checked and gnome 3.6 made this default that way long time ago: https://github.com/GNOME/gsettings-desktop-schemas/commit/19fe99f2f9ed8ebc77fd28465906db33c4782604
what gnome does is absolutely irrelevant.
Of course, that's true. I just wanted to show reason why they did this (some apps, mostly graphic), nothing more. I hope you get my point. As a graphic designer I'm completely unaware of ability to block shortcuts or set something inside system settings because my app isn't working. Last app: https://docs.krita.org/Polygonal_Selection_Tool If you take a glimpse @ google you will find, that most questions how to disable alt+click and some how to enable it back: https://www.google.pl/search?num=100&safe=off&client=firefox-b&q=super%2Bclick+alt%2Bclick+move+window&oq=super%2Bclick+alt%2Bclick+move+window Anyway thank you for your point.
I've started thread about this: https://forum.kde.org/viewtopic.php?f=83&t=139423
Created attachment 109127 [details] KWin KDE 3.5.10 It's perfectly possible in KDE 3.5.10. So, in KDE 5 this setting is no longer present? Wow. It must suck.
It is present with ALT is default. You have to dig inside settings, not really intuitive to find without googling.
*** This bug has been marked as a duplicate of bug 399375 ***