User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0

Steps to reproduce:

Open a webpage with a 3D canvas (WebGL context) on nightly, and resize the page dynamically.

Test page : https://threejs.org/examples/#webgl_materials_standard

(Setup: Nightly on Windows 10, Nvidia RTX 2060 card, And Preview Nightly on Android 10, OnePlus 6)

Actual results:

The content of the canvas, which previously resized correctly, now renders on a streched zone instead of adapting the ratio as it used to.

The size of the canvas is OK, but the rendered portion is stretched. It looks like a size setting problem, or more likely a problem when someone sets the webgl context viewport.

This bug happens on Desktop Nightly and on Preview Nighlty but not on regular stable-branch Firefox.

Expected results:

The aspect ratio of the drawn area should have adapted to the new canvas size.

(See stable Firefox or Chrome to compare)

I could not reproduce this issue - when loading the provided link, I get a crash on the latest Nightly 74.0a1 (Bug 1608311). I'm going to set the component Canvas: WebGL so that someone with more knowledge in this area will take a look over this.

Able to reproduce this issue in Nightly 74.0a1 (2020-01-21), but not Beta 73.0b7 on Windows 10.

Cannot reproduce this issue in a fresh profile in Nightly build 20200121215203.

For the issue to be reproduced, this prefs.js file needs to be copied into the profile.

The only thing you need from the prefs file is to enable webrender.

Regression range

https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=5cbf5bb11c585d69321b420f67ffb62544141427&tochange=e1b0906509efa6433978b9f980ec639b7eafbc6d

-> bug 1477756

Zooming also causes letterboxing, so I suspect a compositing interaction, not a glViewport issue.

We never set mResetLayer = true like we used to, so WebRender thinks it's the same size as ever.

We should write a reftest.

https://hg.mozilla.org/mozilla-central/rev/8c876eb8651e

Able to reproduce this issue in Nightly 74.0a1 (2020-01-17). No longer able to reproduce in 74.0a1 (2020-01-24). All tests performed in Windows 10.