From 01e813736810d4afade739af76893f2d9ff1f93a Mon Sep 17 00:00:00 2001
From: Jonathan Kew <jkew@mozilla.com>
Date: Wed, 23 May 2018 11:57:42 -0700
Subject: Update dimensions early in ClearTarget.

---
 dom/canvas/CanvasRenderingContext2D.cpp | 14 +++++++++-----
 dom/canvas/CanvasRenderingContext2D.h   |  5 ++++-
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/dom/canvas/CanvasRenderingContext2D.cpp b/dom/canvas/CanvasRenderingContext2D.cpp
index f4c4259f6..18af28e9f 100644
--- a/dom/canvas/CanvasRenderingContext2D.cpp
+++ b/dom/canvas/CanvasRenderingContext2D.cpp
@@ -1862,8 +1862,6 @@ CanvasRenderingContext2D::GetHeight() const
 NS_IMETHODIMP
 CanvasRenderingContext2D::SetDimensions(int32_t aWidth, int32_t aHeight)
 {
-  ClearTarget();
-
   // Zero sized surfaces can cause problems.
   mZero = false;
   if (aHeight == 0) {
@@ -1874,14 +1872,14 @@ CanvasRenderingContext2D::SetDimensions(int32_t aWidth, int32_t aHeight)
     aWidth = 1;
     mZero = true;
   }
-  mWidth = aWidth;
-  mHeight = aHeight;
+
+  ClearTarget(aWidth, aHeight);
 
   return NS_OK;
 }
 
 void
-CanvasRenderingContext2D::ClearTarget()
+CanvasRenderingContext2D::ClearTarget(int32_t aWidth, int32_t aHeight)
 {
   Reset();
 
@@ -1889,6 +1887,12 @@ CanvasRenderingContext2D::ClearTarget()
 
   SetInitialState();
 
+  // Update dimensions only if new (strictly positive) values were passed.
+  if (aWidth > 0 && aHeight > 0) {
+    mWidth = aWidth;
+    mHeight = aHeight;
+  }
+
   // For vertical writing-mode, unless text-orientation is sideways,
   // we'll modify the initial value of textBaseline to 'middle'.
   RefPtr<nsStyleContext> canvasStyle;
diff --git a/dom/canvas/CanvasRenderingContext2D.h b/dom/canvas/CanvasRenderingContext2D.h
index d5dff8f3b..848b3ee08 100644
--- a/dom/canvas/CanvasRenderingContext2D.h
+++ b/dom/canvas/CanvasRenderingContext2D.h
@@ -669,8 +669,11 @@ protected:
 
   /**
    * Disposes an old target and prepares to lazily create a new target.
+   *
+   * Parameters are the new dimensions to be used, or if either is negative,
+   * existing dimensions will be left unchanged.
    */
-  void ClearTarget();
+  void ClearTarget(int32_t aWidth = -1, int32_t aHeight = -1);
 
   /*
    * Returns the target to the buffer provider. i.e. this will queue a frame for
-- 
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