Altdrag is efficient. But I want to use mouse left button with other key like Ctrl to resize
Because it's pretty difficult to resize by altdrag with mouse right on a laptop trackpad.
It's very easy to do this in some wms. Is it possible with KDE?
It can be changed in the system settings, but only supports Alt and Meta/Windows keys. The mouse buttons can be freely assigned to different actions.
KDE > System Settings > Window Management > Window Behavior > [Tab] Windows Actions > [Section] Inner Window, Titlebar & Frame > [Droplist] Modifier Key
See also this bug report, where the default modifier key will be changed to Meta in the next kwin release: https://bugs.kde.org/show_bug.cgi?id=399375