Some symbols disappear in globe projection between zoom levels 5 and 6

[osvodef]

mapbox-gl-js version: 2.13.0

browser: Chrome 111

Description

Symbols of these types:

1. Line labels
2. Point labels with variable anchors
3. Symbols with both text-variable-anchor and icon-text-fit set

are not rendered in globe projection between GLOBE_ZOOM_THRESHOLD_MIN and GLOBE_ZOOM_THRESHOLD_MAX.

Steps to Trigger Behavior

1. Go to https://codepen.io/osvodef/pen/povExxg
2. Zoom in

Link to Demonstration

https://codepen.io/osvodef/pen/povExxg

Expected Behavior

All labels stay visible while crossing zoom levels 5–6

Actual Behavior

3 labels disappear, 1 stays

Additional info

I did a bit of detective work myself on this issue, hope this helps.

I tracked the bug down to symbol vertex shaders and u_zoom_transition uniform. Forcing its value to 0 makes the labels appear, but on wrong positions on the screen, as if no globe→mercator transition is happening.

In the shaders, I found this line that hides the labels when u_zoom_transition > 0:

#if defined(PROJECTED_POS_ON_VIEWPORT) && defined(PROJECTION_GLOBE_VIEW)
    projection_transition_fade = 1.0 - step(EPSILON, u_zoom_transition);
#endif

However, just removing this line doesn’t fix the issue since the screen coordinates calculated by the shader are also wrong when u_zoom_transition > 0, usually putting the symbol completely off-screen.

The projection_transition_fade line suggests that this was done intentionally, however, I couldn’t find any relevant discussion in PRs/issues here so I decided to report it anyway.