On some devices, hit testing is not working properly #36817
Description
On a Pinephone Pro running Mobian, I found that I have to tap "above" links to click them. I could not reproduce the issue on desktop Linux with or without -Z convert-mouse-to-touch
Looking at the HiDPI and device per pixel ratio, I found that:
Pinephone
- window.devicePixelRatio = 2
- [servoshell::desktop::headed_window] set_toolbar_height to 52.0 with 1.0 hidpi scale factor
- [servoshell::desktop::app] window scale factor changed to 2, setting egui zoom factor to 1
Linux Desktop
- window.devicePixelRatio = 1.25
- [servoshell::desktop::headed_window] set_toolbar_height to 52.0 with 1.25 hidpi scale factor
- [servoshell::desktop::app] window scale factor changed to 1.25, setting egui zoom factor to 1
The difference between the window.dPR and the hidpi scale factor on the Pinephone is suspicious. Using the --device-pixel-ratio=2 command line option on the PinePhone doesn't help with the hit testing bug.
Since hit testing seems to be done in webrender, could there be some race between our initialization of webrender and winit getting the actual value from wayland? We initialize the Servo instance, compositor and WebRender before we get the "scale factor changed" event from winit.