@variablepulserate I think there is definitely a trend element; I don't see it so strongly that way because UIs don't seem to have been around long enough for us to observe the cyclical nature that we see in fashion (slim fit -> baggy -> slim -> baggy). In UIs the major styles that come to my mind are
- CDE: beveled (some attempt at 3D), colourful
- Windows pre-XP, Mac Platinum: very grey but still with clear window borders, buttons look like buttons.
- Early mobile: whatever could work on 320 pixels.
- Windows XP, Mac Aqua: very colourful, loud, 3D-esque with gradients (scrollbars on XP and Aqua, the close/minimise/maximise buttons on Aqua, 3D dock and Time Machine from Leopard onwards). More 3D emphasis in Vista, pulling back in Windows 7.
- Early Android, iOS: pretty skeumorphic (I think iOS 7 was the peak?)
- Mac OS Lion: the old scrollbars are replaced with thin grey lines; every version of Mac OS after that seems to get progressively flatter.
- Windows 8: the tile design seems to mark the start of the flat design era; Windows 10 and Windows 11 seems to continue this trend.
So we haven't gotten to the point of developing on, for example, the CDE-style buttons-look-like-buttons approach or the loud (almost maximalist) Windows XP/Mac Aqua style yet. It looks like flat design is here to stay for the moment.
All off the top of my head though.