Bug 1566422 - Avoid unnecessary coordinate conversions when showing tooltips, r=emilio

We currently start with screen-relative coordinates, translate them to
widget-relative coordinates, and then translate them back to screen-relative
coordinates when actually showing the tooltip in XULBrowserWindow.showTooltip().
There's no reason for the extra conversions, so we can just send screen-relative
coordinates directly.

Since the widget origin for out-of-process frames is the origin of the frame
itself (instead of the tab, which is the case for in-process frames), the
screen-to-widget conversion was incorrect, and was causing a bug in how the
tooltip was being positioned. Avoiding that conversion altogether also fixes
that bug.

Differential Revision: https://phabricator.services.mozilla.com/D86750

browser/base/content/browser.js

@@ -5024,29 +5024,10 @@ var XULBrowserWindow = {
       return;
     }
 
-    // The x,y coordinates are relative to the <browser> element using
-    // the chrome zoom level.
     let elt = document.getElementById("remoteBrowserTooltip");
     elt.label = tooltip;
     elt.style.direction = direction;
-
-    let screenX;
-    let screenY;
-
-    if (browser instanceof XULElement) {
-      // XUL element such as <browser> has the `screenX` and `screenY` fields.
-      // https://searchfox.org/mozilla-central/source/dom/webidl/XULElement.webidl
-      screenX = browser.screenX;
-      screenY = browser.screenY;
-    } else {
-      // In case of HTML element such as <iframe> which RDM uses,
-      // calculate the coordinate manually since it does not have the fields.
-      const componentBounds = browser.getBoundingClientRect();
-      screenX = window.screenX + componentBounds.x;
-      screenY = window.screenY + componentBounds.y;
-    }
-
-    elt.openPopupAtScreen(screenX + x, screenY + y, false, null);
+    elt.openPopupAtScreen(x, y, false, null);
   },
 
   hideTooltip() {

docshell/base/nsDocShellTreeOwner.cpp

@@ -1287,17 +1287,9 @@ void ChromeTooltipListener::sTooltipCallback(nsITimer* aTimer,
 
       if (textFound && (!self->mTooltipShownOnce ||
                         tooltipText != self->mLastShownTooltipText)) {
-        LayoutDeviceIntPoint screenDot = widget->WidgetToScreenOffset();
-        double scaleFactor = 1.0;
-        if (presShell->GetPresContext()) {
-          nsDeviceContext* dc = presShell->GetPresContext()->DeviceContext();
-          scaleFactor = double(AppUnitsPerCSSPixel()) /
-                        dc->AppUnitsPerDevPixelAtUnitFullZoom();
-        }
-        // ShowTooltip expects widget-relative position.
-        self->ShowTooltip(self->mMouseScreenX - screenDot.x / scaleFactor,
-                          self->mMouseScreenY - screenDot.y / scaleFactor,
-                          tooltipText, directionText);
+        // ShowTooltip expects screen-relative position.
+        self->ShowTooltip(self->mMouseScreenX, self->mMouseScreenY, tooltipText,
+                          directionText);
         self->mLastShownTooltipText = std::move(tooltipText);
         self->mLastDocshell = do_GetWeakReference(
             self->mPossibleTooltipNode->OwnerDoc()->GetDocShell());