It seems that the Aurorae engine in Plasma 5 is showing hover button variants for deactivated buttons. This is easy to see with any Aurorae-based window decoration that has visually distinct (or invisible/missing) deactivated maximize button, such as Simple Glass.

• Go to System Settings / Application Style / Window Decorations
• Install an appropriate Aurorae-based window decoration theme, such as Simple Glass
• Select the theme and Apply
• Open kcalc (Scientific Calculator)
• Note that kcalc's maximize button is deactivated/absent
• Mouse over the deactivated/absent maximize button and note that the active/inactive hover effect is shown

Wanted to check if this is known before I bugzilla this.

https://bugs.kde.org/show_bug.cgi?id=355438