WindowServer: Add an Overlay class for flicker-free overlay rendering

An Overlay is similar to a transparent window, but has less overhead
and does not get rendered within the window stack. Basically, the area
that an Overlay occupies forces transparency rendering for any window
underneath, which allows us to render them flicker-free.

This also adds a new API that allows displaying the screen numbers,
e.g. while the user configures the screen layout in DisplaySettings

Because other things like drag&drop or the window-size label are not
yet converted to use this new mechanism, they will be drawn over the
screen-number currently.

Userland/Services/WindowServer/Compositor.cpp

@@ -8,6 +8,7 @@
 #include "ClientConnection.h"
 #include "Event.h"
 #include "EventLoop.h"
+#include "MultiScaleBitmaps.h"
 #include "Screen.h"
 #include "Window.h"
 #include "WindowManager.h"
@@ -76,7 +77,7 @@ const Gfx::Bitmap& Compositor::front_bitmap_for_screenshot(Badge<ClientConnectio
     return *m_screen_data[screen.index()].m_front_bitmap;
 }
 
-void Compositor::ScreenData::init_bitmaps(Screen& screen)
+void Compositor::ScreenData::init_bitmaps(Compositor& compositor, Screen& screen)
 {
     auto size = screen.size();
 
@@ -97,13 +98,21 @@ void Compositor::ScreenData::init_bitmaps(Screen& screen)
 
     m_buffers_are_flipped = false;
     m_screen_can_set_buffer = screen.can_set_buffer();
+
+    // Recreate the screen-number overlay as the Screen instances may have changed, or get rid of it if we no longer need it
+    if (compositor.showing_screen_numbers()) {
+        m_screen_number_overlay = compositor.create_overlay<ScreenNumberOverlay>(screen);
+        m_screen_number_overlay->set_enabled(true);
+    } else {
+        m_screen_number_overlay = nullptr;
+    }
 }
 
 void Compositor::init_bitmaps()
 {
     m_screen_data.resize(Screen::count());
     Screen::for_each([&](auto& screen) {
-        m_screen_data[screen.index()].init_bitmaps(screen);
+        m_screen_data[screen.index()].init_bitmaps(*this, screen);
         return IterationDecision::Continue;
     });
 
@@ -143,6 +152,9 @@ void Compositor::compose()
         recompute_occlusions();
     }
 
+    // We should have recomputed occlusions if any overlay rects were changed
+    VERIFY(!m_overlay_rects_changed);
+
     auto dirty_screen_rects = move(m_dirty_screen_rects);
     auto* dnd_client = wm.dnd_client();
     if (!m_last_geometry_label_damage_rect.is_empty() || !m_last_dnd_rect.is_empty() || (m_invalidated_cursor && dnd_client)) {
@@ -517,6 +529,11 @@ void Compositor::compose()
             return is_overlapping;
         }());
 
+        if (!m_overlay_list.is_empty()) {
+            // Render everything to the temporary buffer before we copy it back
+            render_overlays();
+        }
+
         // Copy anything rendered to the temporary buffer to the back buffer
         Screen::for_each([&](auto& screen) {
             auto screen_rect = screen.rect();
@@ -828,6 +845,7 @@ void Compositor::screen_resolution_changed()
 
     init_bitmaps();
     invalidate_occlusions();
+    overlay_rects_changed();
     compose();
 }
 
@@ -914,6 +932,54 @@ void Compositor::change_cursor(const Cursor* cursor)
     }
 }
 
+void Compositor::render_overlays()
+{
+    // NOTE: overlays should always be rendered to the temporary buffer!
+    for (auto& overlay : m_overlay_list) {
+        for (auto* screen : overlay.m_screens) {
+            auto& screen_data = m_screen_data[screen->index()];
+            auto& painter = screen_data.overlay_painter();
+            screen_data.for_each_intersected_flushing_rect(overlay.current_render_rect(), [&](auto& intersected_overlay_rect) {
+                Gfx::PainterStateSaver saver(painter);
+                painter.add_clip_rect(intersected_overlay_rect);
+                painter.translate(overlay.m_current_rect.location());
+                overlay.render(painter, *screen);
+                return IterationDecision::Continue;
+            });
+        }
+    }
+}
+
+void Compositor::add_overlay(Overlay& overlay)
+{
+    VERIFY(!overlay.m_list_node.is_in_list());
+    auto zorder = overlay.zorder();
+    bool did_insert = false;
+    for (auto& other_overlay : m_overlay_list) {
+        if (other_overlay.zorder() > zorder) {
+            m_overlay_list.insert_before(other_overlay, overlay);
+            did_insert = true;
+            break;
+        }
+    }
+    if (!did_insert)
+        m_overlay_list.append(overlay);
+
+    overlay.clear_invalidated();
+    overlay_rects_changed();
+    auto& rect = overlay.rect();
+    if (!rect.is_empty())
+        invalidate_screen(rect);
+}
+
+void Compositor::remove_overlay(Overlay& overlay)
+{
+    auto& current_render_rect = overlay.current_render_rect();
+    if (!current_render_rect.is_empty())
+        invalidate_screen(current_render_rect);
+    m_overlay_list.remove(overlay);
+}
+
 void Compositor::ScreenData::draw_cursor(Screen& screen, const Gfx::IntRect& cursor_rect)
 {
     auto& wm = WindowManager::the();
@@ -944,6 +1010,11 @@ bool Compositor::ScreenData::restore_cursor_back(Screen& screen, Gfx::IntRect& l
     return true;
 }
 
+void Compositor::update_fonts()
+{
+    ScreenNumberOverlay::pick_font();
+}
+
 void Compositor::notify_display_links()
 {
     ClientConnection::for_each_client([](auto& client) {
@@ -966,6 +1037,35 @@ void Compositor::decrement_display_link_count(Badge<ClientConnection>)
         m_display_link_notify_timer->stop();
 }
 
+void Compositor::invalidate_current_screen_number_rects()
+{
+    for (auto& screen_data : m_screen_data) {
+        if (screen_data.m_screen_number_overlay)
+            screen_data.m_screen_number_overlay->invalidate();
+    }
+}
+
+void Compositor::increment_show_screen_number(Badge<ClientConnection>)
+{
+    if (m_show_screen_number_count++ == 0) {
+        Screen::for_each([&](auto& screen) {
+            auto& screen_data = m_screen_data[screen.index()];
+            VERIFY(!screen_data.m_screen_number_overlay);
+            screen_data.m_screen_number_overlay = create_overlay<ScreenNumberOverlay>(screen);
+            screen_data.m_screen_number_overlay->set_enabled(true);
+            return IterationDecision::Continue;
+        });
+    }
+}
+void Compositor::decrement_show_screen_number(Badge<ClientConnection>)
+{
+    if (--m_show_screen_number_count == 0) {
+        invalidate_current_screen_number_rects();
+        for (auto& screen_data : m_screen_data)
+            screen_data.m_screen_number_overlay = nullptr;
+    }
+}
+
 bool Compositor::any_opaque_window_above_this_one_contains_rect(const Window& a_window, const Gfx::IntRect& rect)
 {
     bool found_containing_window = false;
@@ -992,6 +1092,50 @@ bool Compositor::any_opaque_window_above_this_one_contains_rect(const Window& a_
     return found_containing_window;
 };
 
+void Compositor::overlays_theme_changed()
+{
+    for (auto& overlay : m_overlay_list)
+        overlay.theme_changed();
+    overlay_rects_changed();
+}
+
+void Compositor::overlay_rects_changed()
+{
+    if (m_overlay_rects_changed)
+        return;
+    m_overlay_rects_changed = true;
+    m_invalidated_any = true;
+    invalidate_occlusions();
+    for (auto& rect : m_overlay_rects.rects())
+        invalidate_screen(rect);
+}
+
+void Compositor::recompute_overlay_rects()
+{
+    // The purpose of this is to gather all areas that we will render over
+    // regular window contents. This effectively just forces those areas to
+    // be rendered as transparency areas, which allows us to render these
+    // flicker-free.
+    m_overlay_rects.clear_with_capacity();
+    for (auto& overlay : m_overlay_list) {
+        auto& render_rect = overlay.rect();
+        m_overlay_rects.add(render_rect);
+
+        // Save the rectangle we are using for rendering from now on
+        overlay.did_recompute_occlusions();
+
+        // Cache which screens this overlay are rendered on
+        overlay.m_screens.clear_with_capacity();
+        Screen::for_each([&](auto& screen) {
+            if (render_rect.intersects(screen.rect()))
+                overlay.m_screens.append(&screen);
+            return IterationDecision::Continue;
+        });
+
+        invalidate_screen(render_rect);
+    }
+}
+
 void Compositor::recompute_occlusions()
 {
     auto& wm = WindowManager::the();
@@ -1007,7 +1151,16 @@ void Compositor::recompute_occlusions()
         return IterationDecision::Continue;
     });
 
-    dbgln_if(OCCLUSIONS_DEBUG, "OCCLUSIONS:");
+    if (m_overlay_rects_changed) {
+        m_overlay_rects_changed = false;
+        recompute_overlay_rects();
+    }
+
+    if constexpr (OCCLUSIONS_DEBUG) {
+        dbgln("OCCLUSIONS:");
+        for (auto& rect : m_overlay_rects.rects())
+            dbgln("  overlay: {}", rect);
+    }
 
     auto& main_screen = Screen::main();
     if (auto* fullscreen_window = wm.active_fullscreen_window()) {
@@ -1127,6 +1280,13 @@ void Compositor::recompute_occlusions()
                 return IterationDecision::Continue;
             });
 
+            if (!m_overlay_rects.is_empty() && m_overlay_rects.intersects(visible_opaque)) {
+                // In order to render overlays flicker-free we need to force these area into the
+                // temporary transparency rendering buffer
+                transparency_rects.add(m_overlay_rects.intersected(visible_opaque));
+                visible_opaque = visible_opaque.shatter(m_overlay_rects);
+            }
+
             bool have_opaque = !visible_opaque.is_empty();
             if (!transparency_rects.is_empty())
                 have_transparent = true;