Bug 1715603 - part 2: Make `AutoScrollChild` not start autoscroll if a modifier key is pressed r=Gijs

Now, `nsIFrame::HandleEvent` moves selection at middle mouse button down.  This
occurs before dispatching the event into the system event group.  Therefore,
`AutoScrollChild` cannot prevent it.

On the other hand, Chrome does not start autoscroll when a modifier is pressed.
This means that our users may not be able to use middle click with modifiers
if web apps do not call `preventDefault()` as expected.  So, this difference
is a potential risk of web-compat.

Therefore, this patch makes `AutoScrollChild` stop starting autoscroll if
`Shift` key is pressed.

Differential Revision: https://phabricator.services.mozilla.com/D119253

modules/libpref/init/all.js

@@ -221,6 +221,14 @@ pref("general.config.obscure_value", 13); // for MCD .cfg files
 pref("general.warnOnAboutConfig", true);
 #endif
 
+// Whether middle button click with a modifier key starts to autoscroll or
+// does nothing.
+pref("general.autoscroll.prevent_to_start.shiftKey", true); // Shift
+pref("general.autoscroll.prevent_to_start.ctrlKey", false); // Control
+pref("general.autoscroll.prevent_to_start.altKey", false);  // Alt
+pref("general.autoscroll.prevent_to_start.metaKey", false); // Command on macOS
+pref("general.autoscroll.prevent_to_start.osKey", false);   // Windows key on Windows or Super key on Linux
+
 // maximum number of dated backups to keep at any time
 pref("browser.bookmarks.max_backups",       5);
 

toolkit/actors/AutoScrollChild.jsm

@@ -337,6 +337,36 @@ class AutoScrollChild extends JSWindowActorChild {
     this._scrollable.ownerGlobal.requestAnimationFrame(this.autoscrollLoop);
   }
 
+  canStartAutoScrollWith(event) {
+    if (
+      !event.isTrusted ||
+      event.defaultPrevented ||
+      event.button !== 1 ||
+      event.clickEventPrevented()
+    ) {
+      return false;
+    }
+
+    for (const modifier of ["shift", "alt", "ctrl", "meta"]) {
+      if (
+        event[modifier + "Key"] &&
+        Services.prefs.getBoolPref(
+          `general.autoscroll.prevent_to_start.${modifier}Key`,
+          false
+        )
+      ) {
+        return false;
+      }
+    }
+    if (
+      event.getModifierState("OS") &&
+      Services.prefs.getBoolPref("general.autoscroll.prevent_to_start.osKey")
+    ) {
+      return false;
+    }
+    return true;
+  }
+
   handleEvent(event) {
     switch (event.type) {
       case "mousemove":
@@ -345,10 +375,7 @@ class AutoScrollChild extends JSWindowActorChild {
         break;
       case "mousedown":
         if (
-          event.isTrusted &&
-          !event.defaultPrevented &&
-          event.button === 1 &&
-          !event.clickEventPrevented() &&
+          this.canStartAutoScrollWith(event) &&
           !this._scrollable &&
           !this.isAutoscrollBlocker(event.originalTarget)
         ) {

toolkit/content/tests/browser/browser_click_event_during_autoscrolling.js

@@ -143,6 +143,7 @@ add_task(async function() {
         }
       }
 
+      await SpecialPowers.pushPrefEnv({ set: [["middlemouse.paste", true]] });
       await (async function testMouseEventsAtStartingAutoScrolling() {
         info(
           "Waiting autoscroller popup for testing mouse events at starting autoscrolling"
@@ -204,6 +205,111 @@ add_task(async function() {
         await waitForAutoScrollEnd;
       })();
 
+      if (
+        // Bug 1693240: We don't support setting modifiers while posting a mouse event on Windows.
+        !navigator.platform.includes("Win") &&
+        // Bug 1693237: We don't support setting modifiers on Android.
+        !navigator.appVersion.includes("Android") &&
+        // In Headless mode, modifiers are not supported by this kind of APIs.
+        !Cc["@mozilla.org/gfx/info;1"].getService(Ci.nsIGfxInfo).isHeadless
+      ) {
+        await SpecialPowers.pushPrefEnv({
+          set: [
+            ["general.autoscroll.prevent_to_start.shiftKey", true],
+            ["general.autoscroll.prevent_to_start.altKey", true],
+            ["general.autoscroll.prevent_to_start.ctrlKey", true],
+            ["general.autoscroll.prevent_to_start.metaKey", true],
+          ],
+        });
+        for (const modifier of ["Shift", "Control", "Alt", "Meta"]) {
+          if (modifier == "Meta" && !navigator.platform.includes("Mac")) {
+            continue; // Delete this after fixing bug 1232918.
+          }
+          await (async function modifiersPreventToStartAutoScrolling() {
+            info(
+              `Waiting to check not to open autoscroller popup with middle button click with ${modifier}`
+            );
+            await promiseFlushLayoutInContent();
+            let eventsInContent = new ContentEventCounter(browser, [
+              "click",
+              "auxclick",
+              "mousedown",
+              "mouseup",
+              "paste",
+            ]);
+            // Ensure that the event listeners added in the content with accessing
+            // the remote content.
+            await promiseFlushLayoutInContent();
+            await EventUtils.promiseNativeMouseEvent({
+              type: "mousemove",
+              target: browser,
+              atCenter: true,
+            });
+            info(
+              `Waiting to MozAutoScrollNoStart event for the middle button click with ${modifier}`
+            );
+            await EventUtils.promiseNativeMouseEvent({
+              type: "mousedown",
+              target: browser,
+              atCenter: true,
+              button: 1, // middle button
+              modifiers: {
+                altKey: modifier == "Alt",
+                ctrlKey: modifier == "Control",
+                metaKey: modifier == "Meta",
+                shiftKey: modifier == "Shift",
+              },
+            });
+            try {
+              await TestUtils.waitForCondition(
+                () => autoScroller?.state == "open",
+                `Waiting to check not to open autoscroller popup with ${modifier}`,
+                100,
+                10
+              );
+              ok(
+                false,
+                `The autoscroller popup shouldn't be opened by middle click with ${modifier}`
+              );
+            } catch (ex) {
+              ok(
+                true,
+                `The autoscroller popup was not open as expected after middle click with ${modifier}`
+              );
+            }
+            // In the wild, native "mouseup" event occurs after the popup is open.
+            await EventUtils.promiseNativeMouseEvent({
+              type: "mouseup",
+              target: browser,
+              atCenter: true,
+              button: 1, // middle button
+            });
+            await promiseFlushLayoutInContent();
+            await promiseContentTick();
+            await eventsInContent.promiseMouseEvents(
+              ["paste"],
+              `At middle clicking with ${modifier}`
+            );
+            for (let eventType of [
+              "mousedown",
+              "mouseup",
+              "click",
+              "auxclick",
+              "paste",
+            ]) {
+              is(
+                eventsInContent.getCountAndRemoveEventListener(eventType),
+                1,
+                `"${eventType}" event should be fired in the content when a middle click with ${modifier}`
+              );
+            }
+            info(
+              "Waiting autoscroller close for preparing the following tests"
+            );
+          })();
+        }
+      }
+
       async function doTestMouseEventsAtStoppingAutoScrolling({
         aButton = 0,
         aClickOutsideAutoScroller = false,