I think this is the best I can do for a hover vs. click shader effect. Just do it in a shader based on a uniform setting.

assets/shaders/ui_shader.frag

@@ -5,18 +5,25 @@ uniform float u_time;
 uniform float u_time_end;
 uniform sampler2D texture;
 uniform bool is_shape;
+uniform bool hover;
 
-void main() {
-  if(is_shape) {
-    float tick = (u_time_end - u_time) / u_duration;
-    float blink = mix(0.5, 1.0, tick);
-    vec4 color = vec4(blink, blink, blink, 1.0);
-    gl_FragColor = gl_Color * color;
+vec4 blink() {
+  if(hover) {
+    return vec4(0.95, 0.95, 1.0, 1.0);
   } else {
-    vec4 pixel = texture2D(texture, gl_TexCoord[0].xy);
     float tick = (u_time_end - u_time) / u_duration;
     float blink = mix(0.5, 1.0, tick);
-    vec4 color = vec4(blink, blink, blink, 1.0);
-    gl_FragColor = gl_Color * color * pixel;
+    return vec4(blink, blink, blink, 1.0);
   }
 }
+
+void main() {
+  vec4 color = blink();
+
+  if(!is_shape) {
+    vec4 pixel = texture2D(texture, gl_TexCoord[0].xy);
+    color *= pixel;
+  }
+
+  gl_FragColor = gl_Color * color;
+}

boss_fight_ui.cpp

@@ -139,12 +139,12 @@ namespace gui {
     $overlay.render(window);
   }
 
-  bool BossFightUI::mouse(float x, float y) {
-    if($status.mouse(x, y)) {
+  bool BossFightUI::mouse(float x, float y, bool hover) {
+    if($status.mouse(x, y, hover)) {
       dbc::log("STATUS button pressed");
     }
 
-    if($overlay.mouse(x, y)) {
+    if($overlay.mouse(x, y, hover)) {
       $animation.play();
       sound::play("Sword_Hit_1");
       $boss_hit = !$boss_hit;

boss_fight_ui.hpp

@@ -38,7 +38,7 @@ namespace gui {
 
       void init();
       void render(sf::RenderWindow& window);
-      bool mouse(float x, float y);
+      bool mouse(float x, float y, bool hover);
       void bounce_boss(sf::RenderWindow& window);
       bool boss_dead() { return $combat.hp < 0; }
       void configure_sprite();

combat_ui.cpp

@@ -46,7 +46,7 @@ namespace gui {
     init();
   }
 
-  bool CombatUI::mouse(float x, float y) {
-    return $gui.mouse(x, y);
+  bool CombatUI::mouse(float x, float y, bool hover) {
+    return $gui.mouse(x, y, hover);
   }
 }

combat_ui.hpp

@@ -17,7 +17,7 @@ namespace gui {
       void render(sf::RenderWindow& window);
       void update_level(GameLevel &level);
       void set_damage(float percent);
-      bool mouse(float x, float y);
+      bool mouse(float x, float y, bool hover);
       void make_button(std::string name, std::wstring label, Events::GUI event);
   };
 }

debug_ui.cpp

@@ -95,8 +95,8 @@ namespace gui {
     }
   }
 
-  bool DebugUI::mouse(float x, float y) {
-    return $gui.mouse(x, y);
+  bool DebugUI::mouse(float x, float y, bool hover) {
+    return $gui.mouse(x, y, hover);
   }
 
   void DebugUI::update_level(GameLevel &level) {

debug_ui.hpp

@@ -17,7 +17,7 @@ namespace gui {
 
       void init(lel::Cell cell);
       void render(sf::RenderWindow& window);
-      bool mouse(float x, float y);
+      bool mouse(float x, float y, bool hover);
       void debug();
       void update_level(GameLevel &level);
       void spawn(std::string enemy_key);

guecs.cpp

@@ -219,7 +219,7 @@ namespace guecs {
     });
   }
 
-  bool UI::mouse(float x, float y) {
+  bool UI::mouse(float x, float y, bool hover) {
     int action_count = 0;
 
     $world.query<lel::Cell, Clickable>([&](auto ent, auto& cell, auto &clicked) {
@@ -227,10 +227,13 @@ namespace guecs {
           (y >= cell.y && y <= cell.y + cell.h))
       {
         if($world.has<Effect>(ent)) {
-          auto& shader = $world.get<Effect>(ent);
-          shader.run();
+          auto& effect = $world.get<Effect>(ent);
+          effect.$shader->setUniform("hover", hover);
+          effect.run();
         }
 
+        if(hover) return; // kinda gross
+
         if(auto action_data = get_if<ActionData>(ent)) {
           clicked.action(ent, action_data->data);
         } else {

guecs.hpp

@@ -143,7 +143,7 @@ namespace guecs {
 
       void init();
       void render(sf::RenderWindow& window);
-      bool mouse(float x, float y);
+      bool mouse(float x, float y, bool hover);
       void debug_layout(sf::RenderWindow& window);
 
       template <typename Comp>

gui_fsm.cpp

@@ -235,18 +235,24 @@ namespace gui {
         if(mouse->button == sf::Mouse::Button::Left) {
           sf::Vector2f pos = $window.mapPixelToCoords(mouse->position);
           if(in_state(State::NEXT_LEVEL)) {
-            $boss_fight_ui->mouse(pos.x, pos.y);
+            $boss_fight_ui->mouse(pos.x, pos.y, false);
 
             if($boss_fight_ui->boss_dead()) {
               event(Event::STAIRS_DOWN);
             }
           } else {
-            $debug_ui.mouse(pos.x, pos.y);
-            $combat_ui.mouse(pos.x, pos.y);
-            $status_ui.mouse(pos.x, pos.y);
-            $main_ui.mouse(pos.x, pos.y);
+            $debug_ui.mouse(pos.x, pos.y, false);
+            $combat_ui.mouse(pos.x, pos.y, false);
+            $status_ui.mouse(pos.x, pos.y, false);
+            $main_ui.mouse(pos.x, pos.y, false);
           }
         }
+      } else if(const auto* mouse = ev->getIf<sf::Event::MouseMoved>()) {
+        sf::Vector2f pos = $window.mapPixelToCoords(mouse->position);
+        $debug_ui.mouse(pos.x, pos.y, true);
+        $combat_ui.mouse(pos.x, pos.y, true);
+        $status_ui.mouse(pos.x, pos.y, true);
+        $main_ui.mouse(pos.x, pos.y, true);
       }
 
       if(const auto* key = ev->getIf<sf::Event::KeyPressed>()) {

main_ui.cpp

@@ -124,13 +124,13 @@ namespace gui {
     dirty();
   }
 
-  void MainUI::mouse(int x, int y) {
+  void MainUI::mouse(int x, int y, bool hover) {
     if($show_level) {
       $show_level = false;
       $level.world->send<Events::GUI>(Events::GUI::STAIRS_DOWN, $level.player, {});
       $overlay_ui.close_label("middle");
     } else {
-      $overlay_ui.$gui.mouse(x, y);
+      $overlay_ui.$gui.mouse(x, y, hover);
     }
   }
 }

main_ui.hpp

@@ -26,7 +26,7 @@ namespace gui {
 
     MainUI(sf::RenderWindow& window);
 
-    void mouse(int x, int y);
+    void mouse(int x, int y, bool hover);
     void debug();
     void render_debug();
 

ritual_ui.cpp

@@ -113,8 +113,8 @@ namespace gui {
     animation::rotate(*bs.sprite, 20.0);
   }
 
-  bool RitualUI::mouse(float x, float y) {
-    return $gui.mouse(x, y);
+  bool RitualUI::mouse(float x, float y, bool hover) {
+    return $gui.mouse(x, y, hover);
   }
 
   void RitualUI::toggle() {

ritual_ui.hpp

@@ -24,7 +24,7 @@ namespace gui {
       GameLevel $level;
 
       RitualUI(GameLevel level);
-      bool mouse(float x, float y);
+      bool mouse(float x, float y, bool hover);
       void toggle();
       bool is_open();
       void init();

status_ui.cpp

@@ -60,11 +60,11 @@ namespace gui {
     $gui.init();
   }
 
-  bool StatusUI::mouse(float x, float y) {
+  bool StatusUI::mouse(float x, float y, bool hover) {
     if($ritual_ui.is_open()) {
-      return $ritual_ui.mouse(x, y);
+      return $ritual_ui.mouse(x, y, hover);
     } else {
-      return $gui.mouse(x, y);
+      return $gui.mouse(x, y, hover);
     }
   }
 

status_ui.hpp

@@ -20,7 +20,7 @@ namespace gui {
       void select_slot(DinkyECS::Entity ent, std::any data);
       void select_ritual();
       void update_level(GameLevel &level);
-      bool mouse(float x, float y);
+      bool mouse(float x, float y, bool hover);
       void log(std::wstring msg);
       void init();
       void render(sf::RenderWindow &window);