Bug 1689884 - Inform PresShell when tablet-mode changes occur r=emilio

Currently, PresShell is not informed when a system setting involving
Windows 10 Tablet Mode, System Dock Mode, or "Convertible Slate Mode" occur.

As this can change the result of @media (pointer) queries, layout should know
about this so it can react accordingly.

Differential Revision: https://phabricator.services.mozilla.com/D107917

widget/windows/nsWindow.cpp

@@ -5313,6 +5313,21 @@ bool nsWindow::ProcessMessage(UINT msg, WPARAM& wParam, LPARAM& lParam,
             }
           }
         }
+
+        // UserInteractionMode, ConvertibleSlateMode, SystemDockMode may cause
+        // @media(pointer) queries to change, which layout needs to know about
+        //
+        // (WM_SETTINGCHANGE will be sent to all top-level windows, so we
+        //  only respond to the hidden top-level window to avoid hammering
+        //  layout with a bunch of NotifyThemeChanged() calls)
+        //
+        if (mWindowType == eWindowType_invisible) {
+          if (!wcscmp(lParamString, L"UserInteractionMode") ||
+              !wcscmp(lParamString, L"ConvertibleSlateMode") ||
+              !wcscmp(lParamString, L"SystemDockMode")) {
+            NotifyThemeChanged(widget::ThemeChangeKind::MediaQueriesOnly);
+          }
+        }
       }
     } break;