FemtoVG / Windows: Window content lag when resizing window

[teknalb]

Slint v1.6.0
Rust v1.79.0

lag.mp4

[tronical]

I can't quite reproduce this, but I guess the way the GL driver handles surface resizing depends also on the driver here.

I'm curious, is this the same if you do a release build?

[teknalb]

Yes, exactly the same on --release build

[teknalb]

Just wild guess, maybe this reference from iced is related to this issue: iced-rs/iced#1203

[ogoffart]

is the problem hapenning with both the femtovg and skia renderer?

[tronical]

If this also happens with renderer-skia as well as renderer-skia-opengl, then perhaps this is similar to #5791.

[teknalb]

Tests using Slint v1.7.2

renderer-femtovg ------> lags like shown in video
renderer-software ------> no lag
renderer-skia-opengl --> no lag
renderer-skia -----------> weird subtle scaling issue when resizing
renderer-vulkan --------> (panic) windows.rs:32:13: Unable to locate LLVM installation

[tronical]

Thank you for testing. Interesting that this is specific to FemtoVG then - hard to image why... unless this is "simply" a performance issue. When resizing the window with femtovg, do you see also a spike in CPU usage compared to Skia?

[teknalb]

I did a test here: https://uploadnow.io/f/Gc8790p
Looks like femtovg uses a bit more CPU than other ones

[tronical]

True, that's a little higher, but not by that much. I looked at the code I don't failing to find a good explanation here. When resizing with FemtoVG and with Skia OpenGL, we call exactly the same code to resize the surface.

[Enyium]

@teknalb:

renderer-skia -----------> weird subtle scaling issue when resizing

Maybe like in #5765?

[lmaxyz]

@teknalb
Is it still reproduces if you add some real-time animation (for example permanent boxes background changing)?

I ran into strange things, that have led me maybe there is something like slint's eco mode, and window resizing not captured as some interaction with window content, so call for screen update is not invoked in time.
For example, in some environment "real-time" animation has lags if you don't interact with window content, but if you do flicking or scrolling lags disappears.