Bug 1320531 - Check whether DPI appears to have changed before calling ChangedDPI + ResetLayout while dragging windows. r=emk

widget/windows/nsWindow.cpp

@@ -5653,13 +5653,16 @@ nsWindow::ProcessMessage(UINT msg, WPARAM& wParam, LPARAM& lParam,
       FinishLiveResizing(MOVING);
       if (WinUtils::IsPerMonitorDPIAware()) {
         // Sometimes, we appear to miss a WM_DPICHANGED message while moving
-        // a window around. Therefore, call ChangedDPI and ResetLayout here,
+        // a window around. Therefore, call ChangedDPI and ResetLayout here
-        // which causes the prescontext and appshell window management code to
+        // if it appears that the window's scaling is not what we expect.
+        // This causes the prescontext and appshell window management code to
         // check the appUnitsPerDevPixel value and current widget size, and
         // refresh them if necessary. If nothing has changed, these calls will
         // return without actually triggering any extra reflow or painting.
-        ChangedDPI();
+        if (WinUtils::LogToPhysFactor(mWnd) != mDefaultScale) {
-        ResetLayout();
+          ChangedDPI();
+          ResetLayout();
+        }
       }
       break;