From 2ec500123f25ef70d13f03a6e4efe0f4c68335f5 Mon Sep 17 00:00:00 2001
From: Jeff Gilbert <jgilbert@mozilla.com>
Date: Wed, 21 Apr 2021 01:03:32 +0000
Subject: [PATCH] Bug 1706510 - If no frontbuffer, reuse BindDefaultFBForRead.
 r=lsalzman

Don't bind mDefaultFB->mFB directly, since it might be multisampled.

Differential Revision: https://phabricator.services.mozilla.com/D112881
---
 dom/canvas/WebGLContext.cpp | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/dom/canvas/WebGLContext.cpp b/dom/canvas/WebGLContext.cpp
index 51f96c7b454e..16160c9e6b17 100644
--- a/dom/canvas/WebGLContext.cpp
+++ b/dom/canvas/WebGLContext.cpp
@@ -991,13 +991,6 @@ bool WebGLContext::FrontBufferSnapshotInto(Range<uint8_t> dest) {
   if (!IsWebGL2()) {
     fbTarget = LOCAL_GL_FRAMEBUFFER;
   }
-
-  gl->fBindFramebuffer(fbTarget,
-                       front->mFb ? front->mFb->mFB : mDefaultFB->mFB);
-  if (pboWas) {
-    BindBuffer(LOCAL_GL_PIXEL_PACK_BUFFER, nullptr);
-  }
-
   auto reset2 = MakeScopeExit([&] {
     DoBindFB(readFbWas, fbTarget);
     if (pboWas) {
@@ -1005,6 +998,20 @@ bool WebGLContext::FrontBufferSnapshotInto(Range<uint8_t> dest) {
     }
   });
 
+  if (front->mFb) {
+    gl->fBindFramebuffer(fbTarget, front->mFb->mFB);
+  } else {
+    if (!BindDefaultFBForRead()) {
+      gfxCriticalError() << "BindDefaultFBForRead failed";
+      return false;
+    }
+  }
+  if (pboWas) {
+    BindBuffer(LOCAL_GL_PIXEL_PACK_BUFFER, nullptr);
+  }
+
+  // -
+
   const auto& size = front->mDesc.size;
   const size_t stride = size.width * 4;
   MOZ_ASSERT(dest.length() == stride * size.height);