GNOME and gestures, Part 2: HdyLeaflet

This is part 2 of a mini-series. Part 1.


Shortly after the WebKit gesture was merged, I started experimenting with making this gesture more widely available. The first step was porting it to Vala and decoupling from WebKit. Since I wrote this part of the gesture tracker from scratch anyway, it was simple and straightforward. The resulting playground project was also used as a convenient place to quickly iterate on the WebKit gesture itself. Later I also reimplemented rendering to match the WebKit one. Here’s how it looked at various points of time:

Other than that, I used the swipe tracker to make a few more demos for Tobias Bernard:

Check out his GUADEC talk showcasing the second demo! :)

At the same time, I started integrating it into libhandy by supporting back/forward swipe in HdyLeaflet. And there I hit four problems:

1. Transitions and visible-child

A folded HdyLeaflet, just like GtkStack, shows one of its children at any given moment, even during child transitions. The second visible child during transitions is just a screenshot. But which child is “real” and which is a screenshot? Turns out the real child is the destination one, meaning the widget switches its visible child when the animation starts. It isn’t a problem if the animation is quick and time-based, but becomes very noticeable with a gesture. Additionally, it means that starting and cancelling a gesture switches the visible child two time.

One solution would be only switching the visible child at the end of the animation (or not at all if it was canceled). The problem is that it’s a major behavior change: applications that listen to visible-child to know when to update the widgets, or sync the property between two leaflets will break.

Another solution would be to draw both children during transitions, but it still means that visible-child changes two times if the gesture was canceled. The problem here is similar: applications wouldn’t expect the other child to still be drawn, but at least it’s just a visual breakage. And it still means that starting and canceling the gesture would mean two visible-child changes.

The second solution may sound better, and yet the current WIP code uses the first one.

2. Visuals

Leaflet had many issues in this area, such as over transition not making sense spatially and bottom widget being visible through the top widget. Additionally, Adrien liked the drop shadow and dimming in WebKit and the demo and wanted to have it in leaflet as well. :)

The first issue was solved by splitting the transition into over and under and clipping the bottom child. Similarly, I implemented shadow and dimming, though it’s pending on those transition types for mode transitions being merged first, so that the shadow can also be added to those, so that it’s consistent.

I’m also not happy with how the dimming and shadow are implemented, neither here nor in WebKit: it’s custom drawing with hardcoded values. Ideally, this needs to be controlled from CSS somehow. GTK itself uses gadgets for things like this (for example, the overshoot effect in GtkScrolledWindow), but that API is private. Having dimming and drop shadow widgets is an overkill, at least until GTK 4 arrives and makes GtkWidget instantiable. Maybe foreign drawing could work…

3. Syncing animation

Often, GTK applications have two leaflets: one in the window’s content area and one in titlebar. Their visible child is always changed at the same time, so it looks like they are one tall leaflet spanning both titlebar and content. This still needs to work with the gesture. And while it’s easy to make nice-looking throwaway demos that do this, syncing actual HdyLeaflets has to be a proper API.

Initially I suggested what I thought was a nice solution with having swipe tracker as a public object and connecting multiple widgets to it. Benjamin Otte and other people immediately pointed out many problems with it, so I researched how other platforms do it. The answer is simple: most platforms don’t. :)

Android has a rather silly way to sync multiple widgets together, but it’s rarely needed, as app bars are just widgets, so they can be packed into a ViewPager without a need to sync two pagers together.

Another constraint is that the solution must not expose animation progress as a write-able property, so it must not be possible to set this value to something arbitrary and get the transition stuck.

4. Interaction with GtkScrolledWindow

GTK event propagation works in two phases: capture and bubble. Widgets can connect to event signal and receive events on bubble phase. Then they return a value to either stop the event or propagate it further. More recently, GTK added various event controllers that allow choosing the phase where they run. With GTK_PHASE_CAPTURE it’s possible to handle events on the capture phase… But their signals don’t support fine-grained stopping/propagation, i.e. don’t have return values (they do in GTK4 though).
All in all, it means that there’s no way to get an event on capture phase and stop it atbitrarily…

Except there is, it’s private and it’s used by GtkScrolledWindow. This widget captures scroll events and stops some of them. For example, if the scrolled window has a vertical scrollbar, but not horizontal, it stops vertical scrolling events and propagates horizontal scrolling. This is harmless, but it also always stops events with is_stop set to TRUE, meaning a leaflet containing a GtkScrolledWindow will get stuck at the end of the gesture. So every single way of receiving events fails in a different and exciting way.

This last issue made me hate life and put the project on a long hiatus.

More demos

A while later while doing another demo (more on that in the next post) I discovered the horrible workaround: the private function for capturing events in GTK has a very simple implementation, so it’s easy to set this handler manually. And of course, with this workaround it just works. This solves the issue #4.

For the issue #3 I made a crude solution similar to already existing HdyHeaderGroup: HdySwipeable and HdySwipeGroup. It’s an RFC at this point, so criticism is welcome.

That allowed me to make a fully working (though still buggy) prototype of swipeable leaflet:

(Yes, that’s a bug there on 0:21)

Another visible problem in the video is that HdyHeaderGroup showing and hiding buttons doesn’t really work with the gesture. One possible solution here would be to show all buttons on all headerbars when folded, but that would once again involve an API break.

The (still very messy) code is here. Even though it’s not ready yet, Shortwave app already makes use of it:

Swipeable leaflet in Shortwave

This also uncovered a crash when a leaflet is created in unfolded state. Oops.

Thanks Felix Haecker for testing, and once again Tobias Bernard for feedback and suggestions while iterating on it.

GNOME and gestures, Part 1: WebKitGTK

This is part 1 of a mini-series. Part 2.

Re-publishing, the original post is too old to show up on Planet GNOME at this point. It was published on August, 8th initially.


Swipe gesture in Epiphany

I’m a big fan of responsive touchpad gestures. For the last half a year (mostly January, February and during the summer) I’ve been working on improving gestures in many areas throughout GNOME. In this series I will do a (belated) overview.

Back/Forward Swipe

Late in the 3.32.x cycle, I saw a commit by Jan-Michael Brummer adding a back/forward swipe to Epiphany. It was really nice to finally have gestures, but it didn’t have any visual feedback. Less importantly, the direction was reversed, as if when scrolling with Natural Scrolling being off. I wanted to give a shot at improving it.

A proper gesture would have to “stick to finger”, showing screenshot of the previous or next page during the gesture, more or less what Safari does on macOS. Specifically, Epiphany would have to take screenshot of every page that is added into back/forward history, show it while the gesture is performed, then continue showing it until the next page loads enough to replace it. Unfortunately, this isn’t really possible to achieve in Epiphany itself: while WebKit does provide API to take snapshots, there’s no way to know when the previous/next page has loaded “enough”.

So I started looking into WebKit instead, where I found out that Safari’s gesture is actually implemented right there! Most parts are present, but the code was not cross-platform. So I started slowly adapting it for GTK. For the most part, the reusable code was a large part of the back end: page snapshot store and snapshot removal logic. That code is now shared between the platforms. The other parts, like actual event processing and drawing, had to be written from scratch.

One interesting detail about the gesture is that it doesn’t actually use gesture events! Libinput defines swipe gestures as synchronous movement of three or more fingers in the same direction. Mac gesture API is even more strict: it’s three fingers only, with four-finger swipes being reserved for the OS. But the gesture uses two fingers, how is this possible? Turns out it actually uses scroll events instead. (That’s also why it works with Magic Mouse in macOS, even though the code does not special-case anything for it)

When using scroll events, one has to be very careful. Touchpads generate scroll events continuously, in GTK it means that these gestures have GDK_SCROLL_SMOOTH scroll direction. At the very end of the scrolling, there will be a special event with is_stop field set to TRUE, which is used as a signal to start kinetic scrolling or, in our case, to end a swipe.

But there are other input devices, for example, mice. Most mice have “clicky” wheels that generate scroll events with direction instead of deltas. These events are impossible to use for swipes, so there’s no point in even trying to handle them. But there are also mice with freely scrolling wheels which generate the same events as touchpad, except there’s no event with is_stop == TRUE at the end. This means that they can be used to start a swipe, but it will get stuck as soon as the wheel stops. So, these mice have to be skipped too. Then there are touch mice where scrolling probably works same as on touchpad. I suspect swiping can work very well with them, same as it does with Magic Mouse on macOS, but there’s no way to distinguish these kinds of mice, at least as far as I know, so I had to disable it for any mice.

Another problem is that in order to not interfere with actual scrolling, the gesture controller must check whether it is possible to scroll the page first. Maybe there’s still space to scroll, maybe the page intercepts the scroll events. Then there has to be a threshold so that it’s hard to accidentally trigger the gesture. Thankfully, this part is shared with the Mac gesture. :)

A side effect of using scroll events is that this gesture still works with X11 and/or older semi-mt touchpads for which Libinput normally does not support any gestures.

So, now the gesture sticks to fingers. But there’s still an important bit missing: a snap-back animation. In order for a gesture to feel natural, it should snap back smoothly as soon as you lift your fingers, respecting the momentum. This was a lot easier to do than I expected, and after a few iterations applying Tobias Bernard’s suggestions I had an animation that I’m very satisfied with. How it works:

  • The animation uses easeOutCubic interpolation
  • Duration is naturally calculated as remaining distance divided by velocity, or if the velocity is 0, by a constant value instead
  • After that, duration is multiplied by 3, matching easeOutCubic derivative at t=0. This ensures that initial velocity is same as it was before lifting the fingers
  • Finally, the duration is clamped into [100ms, 400ms] range. This ensures that it’s never too slow or too fast, while still allowing it to respect momentum when possible
  • If the page was swiped less than halfway through the window, there’s a small velocity threshold. If fingers are lifted when not moving, the gesture will be canceled and the page will smoothly slide back. On the other hand, if the page was swiped more than half way through, just lifting the fingers would finish the gesture, and one has to specifically flick back to cancel it
  • If one starts swiping again while the animation is going, it will actually be stopped. This allows to continuously grab and release the page

Interestingly, Mac has a helper function that takes care of all this, and WebKit makes use of it.

Finally, the gesture should look nice. On Mac it uses CoreAnimation for drawing; WebKitGTK has to use Cairo. Since I didn’t have any mockups to work with, I reused Apple’s visuals, consisting of a dimming layer and a long subtle gradient for drop shadow, intending to replace them with something else later. But everybody whom I showed it liked it, so I left it as is.

The end result is that since version 2.24.0, WebKitGTK optionally supports 2-finger swipe gestures on touchpad.

Unfortunately, I was a little too late to enable it in Epiphany 3.32.0, but it was merged into 3.32.1 nevertheless. In 3.34.x, Yelp and Devhelp will also support it. Additionally, it’s enabled in Eolie and Odysseus browsers.

A bit later I also added touchscreen support. That was easy, because WebKit literally generates scroll events for touch scrolling, so it was simply a matter of feeding those events into the gesture controller. Additionally, I had to implement canceling, as all touchscreen gestures have to support it. Since scrolling on touchscreen is 1-finger swipe, the gesture is performed the same way.

This will be available in upcoming WebKitGTK 2.26.x, corresponding to GNOME 3.34.

This can be very disruptive feature in many cases, such as authentication widgets, so applications wanting to use it have to opt in by changing the value of this property.

Pinch Zoom

A smaller change was getting pinch zoom gesture to work on touchpads. Since this gesture was already available on touchscreens, it involved simply feeding touchpad gesture events into the gesture tracker, but the performance is severely lacking on heavy pages. Speeding it up is unfortunately still above my skill level. :)

Pinch zoom is enabled unconditionally, so it already works everywhere where WebKitGTK is used, including but not limited to Geary and documentation view in GNOME Builder.


I want to say thanks to the following people:

  • Michael Catanzaro and Carlos Garcia Campos for code review and helping me with understanding WebKit codebase
  • Tobias Bernard for testing and numerous design suggestions
  • Jonas Dre├čler for testing and feedback, especially on a touchscreen

Part 2 here.