From 2e551acc8d515d5c3585b545b4ae873e70bfe7ee Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Fri, 13 Nov 2020 15:14:05 +0100
Subject: [PATCH] Ensure that rendering of thumbnails work correctly on HiDPI
 displays (issue 9820)

*Note that I wasn't able to reproduce the issue in Firefox, but only in Chromium-browsers.*

The bug, and it's feels almost trivial once you've found it, is that we're not passing the `transform` parameter as intended to `PDFPageProxy.render` when drawing thumbnails on HiDPI displays. Instead the canvas context is, for reasons that I don't even pretent to understand, *manually* scaled in `PDFThumbnailView._getPageDrawContext`, which thus doesn't guarantee that the `baseTransform` property on the `CanvasGraphics`-instances becomes correct.

The solution is really simple though, just handle the `transform` the same way in `PDFThumbnailView.draw` as in `PDFPageView.paintOnCanvas` and things should just work.
---
 web/pdf_thumbnail_view.js | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/web/pdf_thumbnail_view.js b/web/pdf_thumbnail_view.js
index 633f8ab65..0097474f3 100644
--- a/web/pdf_thumbnail_view.js
+++ b/web/pdf_thumbnail_view.js
@@ -229,7 +229,7 @@ class PDFThumbnailView {
   /**
    * @private
    */
-  _getPageDrawContext(noCtxScale = false) {
+  _getPageDrawContext() {
     const canvas = document.createElement("canvas");
     // Keep the no-thumbnail outline visible, i.e. `data-loaded === false`,
     // until rendering/image conversion is complete, to avoid display issues.
@@ -249,10 +249,11 @@ class PDFThumbnailView {
     canvas.style.width = this.canvasWidth + "px";
     canvas.style.height = this.canvasHeight + "px";
 
-    if (!noCtxScale && outputScale.scaled) {
-      ctx.scale(outputScale.sx, outputScale.sy);
-    }
-    return ctx;
+    const transform = outputScale.scaled
+      ? [outputScale.sx, 0, 0, outputScale.sy, 0, 0]
+      : null;
+
+    return [ctx, transform];
   }
 
   /**
@@ -333,7 +334,7 @@ class PDFThumbnailView {
       }
     };
 
-    const ctx = this._getPageDrawContext();
+    const [ctx, transform] = this._getPageDrawContext();
     const drawViewport = this.viewport.clone({ scale: this.scale });
     const renderContinueCallback = cont => {
       if (!this.renderingQueue.isHighestPriority(this)) {
@@ -349,6 +350,7 @@ class PDFThumbnailView {
 
     const renderContext = {
       canvasContext: ctx,
+      transform,
       viewport: drawViewport,
       optionalContentConfigPromise: this._optionalContentConfigPromise,
     };
@@ -393,7 +395,7 @@ class PDFThumbnailView {
 
     this.renderingState = RenderingStates.FINISHED;
 
-    const ctx = this._getPageDrawContext(true);
+    const [ctx] = this._getPageDrawContext();
     const canvas = ctx.canvas;
     if (img.width <= 2 * canvas.width) {
       ctx.drawImage(