qml: Change swipe dismiss gesture to animate opacity instead of position

[espidev]

See rationale on #157, but TLDR we encounter a lot of issues with animating the position of the keyboard.

Here we instead change to animate the opacity, which allows the keyboard retain its dimensions during the animation.

[espidev]

VID_20240313_162116.mp4

[dobey]

As kwin disables the animation in maliit, I wonder if #181 may fix the issue you're seeing under Plasma? It's a smaller change, and it's unclear to me how animating opacity helps here. I still would prefer to just remove the swipe to close anyway, as there are so many problems with it, and the code is a bit complex.

[espidev]

As kwin disables the animation in maliit, I wonder if #181 may fix the issue you're seeing under Plasma? It's a smaller change, and it's unclear to me how animating opacity helps here. I still would prefer to just remove the swipe to close anyway, as there are so many problems with it, and the code is a bit complex.

That PR doesn't seem to change anything, it still glitches in the same way when I swipe down

[ell1e]

I tested this now. In overall it seems like a huge improvement, but I found some nitpicks and one seemingly bigger issue:

The bigger issue: When I put my finger on a letter key, then drag down until the keyboard disappears but keeping my finger pressed on the bezel, and then move my finger back into the screen area and just wildly around and in/out of the screen, often the keyboard will eventually get stuck in hidden mode. At this point it seems impossible to get it to show again, unless I tap "Input Method" in the tray enough to deactivate and reactivate the on-screen keyboard entirely. This means users without that tray icon will probably be stuck.

Update: I tried to repeat this one 👆 and catch it on video, but sadly I failed. I'm not sure why. 😕

The smaller nitpicks:

1. When doing a regular faster downward swipe starting from any letter key, without doing anything weird, and at a reasonably fast speed, the drag distance to hide the keyboard seems too large. When starting e.g. in the vertical middle of the keyboard, it's almost impossible to close it anymore even when trying very hard, since I would need to drag farther than physically possible until I hit the screen bezel. This might be a problem when people use this action a lot, since then you don't want to need to make a huge physical movement straining your fingers. (Possible tweaking issue, probably not a bug.)

   Update: here is a video of it taking really long distance, also it pops back up at the end which seems undesired:

   Long.Dragging.Distance.Required.webm

2. Possible small bug when dragging unusually: When I put my finger such on a top row letter with a long press that one of the small popups show up, and then move my finger still pressed down onto the popup and only then drag down, the distance code seems to become confused. It now requires a much smaller drag distance to close, like it somehow looses its proper reference for how far I dragged. While I would personally appreciate a smaller distance needed in general, this feels like it's probably a bug.

   Update: here is a video of this happening:

   Maliit.Unusually.Short.Dragging.Distance.webm

(I tested this on a Steam Deck with postmarketOS Edge, if somehow relevant.)