One of the realities of many graphics / editing applications is, that the action to drag either an object or one of its handles with a mouse, or with any other pointing-device, needs to be modified in several ways, depending on what, exactly, the user expects the application to do. I have recently encountered a Web-application, which therefore displays in a Web-browser, in which to hold down the Shift-Key while dragging the corner of a rectangle does one thing, holding down the Ctrl-Key while doing so does another, and holding down the Alt-Key performs yet-another action, which in this case was, to draw an ellipse instead of a rectangle. And in this Web-application, the ability to draw circles or ellipses would eventually become important.
The problem with any application that has this as one of the inputs it accepts, is that such applications were never designed with Linux in mind. The reason is the fact that, under Linux desktop-managers, to <Alt>+(LMB-Drag) gets intercepted, and has as effect to drag the entire application window. That Web-application never receives such a combination of inputs.
Fortunately, if the desktop-manager is one of the ‘Plasma 5′ sub-versions, there is a way to change that behaviour. It can be found under:
System Settings -> Application Style -> Widgets Style and Behaviour -> Applications Tab -> (Breeze Theme from Drop-Down) -> (To the right of the theme chosen from the Drop-Down) Configure… -> General Tab -> Windows’ drag mode -> (From the drop-down) ‘…Titlebar Only’.
That last drop-down is not wide enough to show its full text.
The reason this defaults to ‘Anywhere from within the window’, is a fear that might exist, that an application’s window could end up very-incorrectly positioned on the screen, and that a user might not be able to change that position, by the next time he starts the application. I.e., some applications remember their window-position from one session to the next, and it could be screwed up. If this behaviour is changed, <Alt>+(LMB-Drag) will only drag the window, and therefore be intercepted before it reaches the application, if the mouse-pointer can be positioned over the window’s title-bar.
Under KDE-4, ‘back in the old days’, that setting was just a little easier to find:
System Settings -> Window Behaviour -> Window Behaviour -> Window Actions -> Inner Window, Titlebar & Frame -> (Next to ‘Modifier key’) Left Button: -> (From The Drop-Down List) ‘Nothing’