Bug 1676057 - Disable scrollbar buttons when non-native theme is in use on Linux. r=emilio,spohl

Whether scrollbar buttons are shown is currently determined by
LookAndFeel values, and these are exposed through system metric media
features.  LookAndFeel values are process-wide, but which theme is in
use is document-specific.  (It can be the platform's native theme or the
native basic theme, depending on Document::ShouldAvoidNativeTheme.)  So
if nsNativeBasicTheme on Linux needs to not support scrollbar buttons,
we can't do this by forcing the LookAndFeel values to be false.

So instead, we skip creating the native anonymous content for the
scrollbar buttons when the non-native theme is in use on Linux.  There's
no need to support dynamic changes, since whether the non-native theme
is in use for a given document never changes.

Differential Revision: https://phabricator.services.mozilla.com/D96345

gfx/src/nsITheme.h

@@ -223,6 +223,8 @@ class nsITheme : public nsISupports {
    * Should we insert a dropmarker inside of combobox button?
    */
   virtual bool ThemeNeedsComboboxDropmarker() = 0;
+
+  virtual bool ThemeSupportsScrollbarButtons() { return true; }
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsITheme, NS_ITHEME_IID)

layout/xul/nsScrollbarFrame.cpp

@@ -349,7 +349,9 @@ nsresult nsScrollbarFrame::CreateAnonymousContent(
       nodeInfoManager->GetNodeInfo(nsGkAtoms::scrollbarbutton, nullptr,
                                    kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
 
-  {
+  bool createButtons = PresContext()->Theme()->ThemeSupportsScrollbarButtons();
+
+  if (createButtons) {
     AnonymousContentKey key;
     mUpTopButton =
         MakeScrollbarButton(sbbNodeInfo, vertical, /* aBottom */ false,
@@ -357,7 +359,7 @@ nsresult nsScrollbarFrame::CreateAnonymousContent(
     aElements.AppendElement(ContentInfo(mUpTopButton, key));
   }
 
-  {
+  if (createButtons) {
     AnonymousContentKey key;
     mDownTopButton =
         MakeScrollbarButton(sbbNodeInfo, vertical, /* aBottom */ false,
@@ -388,7 +390,7 @@ nsresult nsScrollbarFrame::CreateAnonymousContent(
     mSlider->AppendChildTo(mThumb, false);
   }
 
-  {
+  if (createButtons) {
     AnonymousContentKey key;
     mUpBottomButton =
         MakeScrollbarButton(sbbNodeInfo, vertical, /* aBottom */ true,
@@ -396,7 +398,7 @@ nsresult nsScrollbarFrame::CreateAnonymousContent(
     aElements.AppendElement(ContentInfo(mUpBottomButton, key));
   }
 
-  {
+  if (createButtons) {
     AnonymousContentKey key;
     mDownBottomButton =
         MakeScrollbarButton(sbbNodeInfo, vertical, /* aBottom */ true,

widget/gtk/nsNativeBasicThemeGTK.h

@@ -35,6 +35,7 @@ class nsNativeBasicThemeGTK : public nsNativeBasicTheme {
                          nsIFrame* aFrame, const ComputedStyle& aStyle,
                          const EventStates& aDocumentState, uint32_t aDpiRatio,
                          bool aIsRoot) override;
+  bool ThemeSupportsScrollbarButtons() override { return false; }
 
  protected:
   virtual ~nsNativeBasicThemeGTK() = default;