Elevation profile: raster profile rendering misbehaves

[wonder-sk]

What is the bug or the crash?

Rendering of profiles of raster DEMs sometimes does not work correctly - observable mainly when zoomed in to the point where profile spans across just few pixels of the DEM:

• panning left/right can suddenly change parts of the profile line
• disabling and enabling profile can misplace the profile - after another pan/zoom action it behaves as before

Both issues demonstrated here:

https://user-images.githubusercontent.com/193367/191254488-cef337ad-4b6a-40ef-9b9a-66faced37a86.mp4

Steps to reproduce the issue

1. open a raster DEM + mark it as an elevation surface in layer properties
2. open profile tool and draw a line
3. zoom into the profile so that only few pixels of the raster are visible
4. observe the erratic behavior described above

Versions

QGIS master

Supported QGIS version

• [ ] I'm running a supported QGIS version according to the roadmap.

New profile

• [ ] I tried with a new QGIS profile

Additional context

No response

[AlisterH]

The profile never seems quite right to me. Zoom in and compare values in the profile with values shown in the Value Tool, or measure distances in the map and compare with distances in the profile. In this screenshot 2.2m measured on the map somehow equals 1.7m on the profile:

[AlisterH]

Sorry, my cursor disappeared when I took the screen snip. I was measuring from the left end of the profile line in the map to the closest edge of the first pixel with elevation=12.

[AlisterH]

Possibly related to this: Zoom in on a profile until you get a clearly "stepped" view. Toggle a layer off. Toggle it back on again. Pan with the middle mouse button.

[AlisterH]

Also, when toggling layers on and off in the profile QGIS sometimes gets very confused:

https://github.com/qgis/QGIS/assets/714600/249e1339-210e-4cc3-82b9-9e14a11ae079

https://github.com/qgis/QGIS/assets/714600/21f17ce8-fe13-4088-ad9a-a2620ae87719