Bug 1761839 - Allow user-select: none to override modal dialog selectability. r=sefeng

Instead of enforcing it in C++ use a UA rule instead, so that authors
can override it if needed.

Depends on D142582

Differential Revision: https://phabricator.services.mozilla.com/D142583

layout/generic/nsIFrame.cpp

@@ -4651,12 +4651,6 @@ static bool IsEditingHost(const nsIFrame* aFrame) {
   return element && element->IsEditableRoot();
 }
 
-static bool IsTopmostModalDialog(const nsIFrame* aFrame) {
-  auto* element = Element::FromNodeOrNull(aFrame->GetContent());
-  return element &&
-         element->State().HasState(NS_EVENT_STATE_TOPMOST_MODAL_DIALOG);
-}
-
 static StyleUserSelect UsedUserSelect(const nsIFrame* aFrame) {
   if (aFrame->IsGeneratedContentFrame()) {
     return StyleUserSelect::None;
@@ -4681,14 +4675,10 @@ static StyleUserSelect UsedUserSelect(const nsIFrame* aFrame) {
     return style;
   }
 
-  if (aFrame->IsTextInputFrame() || IsEditingHost(aFrame) ||
-      IsTopmostModalDialog(aFrame)) {
+  if (aFrame->IsTextInputFrame() || IsEditingHost(aFrame)) {
     // We don't implement 'contain' itself, but we make 'text' behave as
     // 'contain' for contenteditable and <input> / <textarea> elements anyway so
     // this is ok.
-    //
-    // Topmost modal dialogs need to behave like `text` too, because they're
-    // supposed to be selectable even if their ancestors are inert.
     return StyleUserSelect::Text;
   }
 

layout/style/res/html.css

@@ -829,6 +829,9 @@ dialog:not([open]) {
  * Avoid doing this if the dialog is explicitly inert though. */
 dialog:not([inert]):-moz-topmost-modal-dialog {
   -moz-inert: none;
+  /* Topmost modal dialog needs to be selectable even though ancestors are
+   * inert, but allow users to override this if they want to. */
+  user-select: text;
 }
 
 dialog:-moz-modal-dialog {

testing/web-platform/meta/html/semantics/interactive-elements/the-dialog-element/modal-dialog-selection.html.ini

@@ -1,3 +0,0 @@
-[modal-dialog-selection.html]
-  ['user-select: none' prevents text from being selected]
-    expected: FAIL