[4.6.3] fixed #28866: Keyboard input broken for most spin boxes under KDE Plasma + Wayland

[Eism]

see #29888

[DmitryArefiev]

Tested LinuxUbuntu24.04.3 LTS. It has a regression with range selection when editing text (unable to select):

Screencast.from.2025-10-21.11-35-20.webm

Hadn't seen comment about regression yet

[igorkorsukov]

Maybe all variables here can be made static, including the final result
or like this

static int is_wayland = -1;
if (is_wayland == -1) {
    QString sessionType = qEnvironmentVariable("XDG_SESSION_TYPE");
    QString platformName = qGuiApp->platformName();
    is_wayland =  sessionType.contains("wayland") || platformName.contains("wayland");
}
return is_wayland;