Instantiating 2 views at the same time is not possible: the second view crashes

[bourdaisj]

Description

Occuring when you instantiate two LocalView (with one render windows per local view).

Where does the issue originate?

Upstream Project (vtk.wasm JS bindings)

Version Information

"trame-vtklocal==1.0.1", "vtk==9.6.20260405.dev0"

Steps to Reproduce

Just run this minimal reproduction script using uv run, dependencies are in shebang:

repro.py

Relevant Log Output

JS console:

Could not startEventLoop for 49

49 is the id of the second render window

[bourdaisj]

@jourdain @jspanchu I would be happy to fix it if you can point me in the right direction

[jspanchu]

Check why it returns failure at https://gitlab.kitware.com/vtk/vtk/-/blob/master/Serialization/Manager/vtkSession.cxx?ref_type=heads#L452.

This function was recently updated but the dev wheels corresponding to the new code have not been published yet.

[jspanchu]

it's due to the per-session updateInProgress counter in vtk-wasm's updateAsync.

I put print statements in code and converted a TRACE log in vtkSession.cxx to a WARNING. What is happening is that rw2 mounts while rw1's updateAsync is still awaiting networkFetchStatus, rw2's updateAsync(49) sees updateInProgress === 2 and returns immediately without fetching or registering render window 49. rw2's update() resolves, rw2's onMounted hook proceeds to startEventLoop(49)... but object 49 was never created, hence Object at id=49 is null, and that is why it could not start event loop for 49

before update id= 1
trame_vtklocal.umd.js:1724 before updateAsync id= 1
trame_vtklocal.umd.js:874 before networkFetchStatus id= 1
trame_vtklocal.umd.js:1778 before update id= 49
trame_vtklocal.umd.js:1724 before updateAsync id= 49
trame_vtklocal.umd.js:1726 updateAsync finished id= 49
trame_vtklocal.umd.js:1780 update finished id= 49
index-DBTNMoYm.js:23 2026-07-14 10:33:23.398 (   0.039s) [          872834]         vtkSession.cxx:439    ERR| Object at id=49 is null
index-DBTNMoYm.js:23 Could not startEventLoop for 49
trame_vtklocal.umd.js:876 networkFetchStatus finished id= 1
trame_vtklocal.umd.js:874 before networkFetchStatus id= 1
trame_vtklocal.umd.js:876 networkFetchStatus finished id= 1
trame_vtklocal.umd.js:1726 updateAsync finished id= 1
trame_vtklocal.umd.js:1780 update finished id= 1

Fix will go to vtk-wasm. This may fix #77 too.

[jspanchu]

Okay, after my fix and adding style="height=100%;" in the VRow of your repro.py, it works.

[jspanchu]

i have fix for the event loop, i want to roll in a fix for #77 in a different commit too and open MR then.

[jourdain]

trame-vtklocal>=1.0.2 fix it