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I've been trying to process scroll-events from a GTK window. I have a custom scrollbar that handles smooth scrolling events and non-smooth scrolling events differently, and simulates smooth scrolling for non-smooth scrolling events. I learned from the docs that GDK sets GdkEventScroll::direction to GDK_SCROLL_SMOOTH for smooth scroll events. However, I noticed that when I scroll the middle mouse button (my mouse does not support smooth scrolling), GTK sends two separate events:

  • one event with direction set to GDK_SCROLL_UP or GDK_SCROLL_DOWN and delta_y set to 0, and
  • one event with direction set to GDK_SCROLL_SMOOTH and delta_y set to 1 or -1.

Before adding the special handing I relied solely on delta_y which happened to work fine for me. Now that I've started treating smooth and non-smooth scrolling differently, I need a reliable way to distinguish between these kinds of events.

  • Is there a flag that I can set to have GDK send only non-smooth scrolling events for non-smooth scrolling, and vice versa? That would be my preferred solution.
  • If the previous one is not possible, is there any way I can tell that two scroll events are actually the same event? I see that there's a time field in the event structure, but I doubt it'll be reliable.

Thank you.

Luke Zhou
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1 Answers1

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You mentioned it yourself: the time field is the key. If Gtk generates multiple events from the same “real” event, the time will be the same.

It has millisecond resolution, so I’m pretty sure you will not see multiple real events on the same millisecond. It will be hard to scroll that fast on the mouse to do it the same millisecond.—I have not checked the code, but Gtk should flatten multiple smooth device events so that multiple of those will not have the same millisecond either.

Gtk will send both types of events for both kind of devices (one real and one fake) so that applications work if they handle only one type.—You can go for the device field to see if you get scrolling from multiple devices.

Out of curiosity: do you have a single device with sends both smooth and non-smooth events!?

Robert Siemer
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