Bug 1812202 - Reduce the max capture dimension by 1px. r=niklas

* Captures at exactly the max dimension (32767) result in a fully transparent image.
* Import these dimensions to use in tests so we have a single source of truth
* Update the same constant in the extension

Differential Revision: https://phabricator.services.mozilla.com/D190221

browser/components/screenshots/ScreenshotsUtils.sys.mjs

@@ -27,8 +27,8 @@ const PanelOffsetY = -8;
 // The max dimension for a canvas is defined https://searchfox.org/mozilla-central/rev/f40d29a11f2eb4685256b59934e637012ea6fb78/gfx/cairo/cairo/src/cairo-image-surface.c#62.
 // The max number of pixels for a canvas is 124925329 or 11177 x 11177.
 // We have to limit screenshots to these dimensions otherwise it will cause an error.
-const MAX_CAPTURE_DIMENSION = 32767;
-const MAX_CAPTURE_AREA = 124925329;
+export const MAX_CAPTURE_DIMENSION = 32766;
+export const MAX_CAPTURE_AREA = 124925329;
 
 export class ScreenshotsComponentParent extends JSWindowActorParent {
   async receiveMessage(message) {
@@ -517,8 +517,8 @@ export var ScreenshotsUtils = {
   },
 
   /**
-   * The max one dimesion for a canvas is 32767 and the max canvas area is
-   * 124925329. If the width or height is greater than 32767 we will crop the
+   * The max dimension of any side of a canvas is 32767 and the max canvas area is
+   * 124925329. If the width or height is greater or equal to 32766 we will crop the
    * screenshot to the max width. If the area is still too large for the canvas
    * we will adjust the height so we can successfully capture the screenshot.
    * @param {Object} rect The dimensions of the screenshot. The rect will be

browser/components/screenshots/tests/browser/browser_screenshots_test_screenshot_too_big.js

@@ -39,11 +39,11 @@ add_task(async function test_screenshot_too_large_cropped() {
 
   ScreenshotsUtils.cropScreenshotRectIfNeeded(rect);
 
-  is(rect.width, MAX_CAPTURE_DIMENSION, "The width is 32767");
+  is(rect.width, MAX_CAPTURE_DIMENSION, "The width is 32766");
   is(
     rect.height,
     Math.floor(MAX_CAPTURE_AREA / MAX_CAPTURE_DIMENSION),
-    "The height is 124925329 / 32767"
+    "The height is 124925329 / 32766"
   );
 
   rect.width = 40000;
@@ -54,7 +54,7 @@ add_task(async function test_screenshot_too_large_cropped() {
   is(
     rect.width,
     MAX_CAPTURE_DIMENSION,
-    "The width was cropped to the max capture dimension (32767)."
+    "The width was cropped to the max capture dimension (32766)."
   );
 
   rect.width = 1;
@@ -65,7 +65,7 @@ add_task(async function test_screenshot_too_large_cropped() {
   is(
     rect.height,
     MAX_CAPTURE_DIMENSION,
-    "The height was cropped to the max capture dimension (32767)."
+    "The height was cropped to the max capture dimension (32766)."
   );
 
   rect.width = 12345;

browser/components/screenshots/tests/browser/head.js

@@ -19,8 +19,9 @@ const TEST_PAGE = TEST_ROOT + "test-page.html";
 const SHORT_TEST_PAGE = TEST_ROOT + "short-test-page.html";
 const LARGE_TEST_PAGE = TEST_ROOT + "large-test-page.html";
 
-const MAX_CAPTURE_DIMENSION = 32767;
-const MAX_CAPTURE_AREA = 124925329;
+const { MAX_CAPTURE_DIMENSION, MAX_CAPTURE_AREA } = ChromeUtils.importESModule(
+  "resource:///modules/ScreenshotsUtils.sys.mjs"
+);
 
 const gScreenshotUISelectors = {
   panelButtons: "#screenshotsPagePanel",

browser/extensions/screenshots/background/takeshot.js

@@ -8,7 +8,7 @@
 
 this.takeshot = (function () {
   const exports = {};
-  const MAX_CANVAS_DIMENSION = 32767;
+  const MAX_CANVAS_DIMENSION = 32766;
 
   communication.register(
     "screenshotPage",