Studio: toolbar cosmetic improvements

- Remove hard-coded button sizes (48×32) from toolbar icons; buttons now use default ImGui sizing, matching the existing copy-camera button.
- Remove the Unload button (available via menu).
- Add ImGui_ColorButtonEx widget with per-corner rounding via ImDrawFlags, enabling segmented button-group visuals.
- Use segmented rounding for the pause/viscous/play triplet: rounded-left, square, rounded-right.
- Make pause and play buttons 40% wider than the viscous button for emphasis.
- Replace the theme combo dropdown with a cycling button.
- Reduce toolbar height from 48px to 34px and vertically center content via WindowPadding.
- Zero out table CellPadding in the toolbar for precise vertical centering.
- Remove the speed combo's inflated vertical padding.

PiperOrigin-RevId: 906853958
Change-Id: I6e4ec7243adb9e0f1f06f8779fa896a6865c46c7

src/experimental/studio/app.cc

@@ -72,13 +72,11 @@ static void SelectParentPerturb(const mjModel* model, mjvPerturb& perturb) {
 }
 
 static constexpr const char* ICON_COPY_CAMERA = platform::ICON_FA_COPY;
-static constexpr const char* ICON_UNLOAD_MODEL = platform::ICON_FA_EJECT;
 static constexpr const char* ICON_RELOAD_MODEL = platform::ICON_FA_REFRESH;
 static constexpr const char* ICON_RESET_MODEL = platform::ICON_FA_UNDO;
 static constexpr const char* ICON_PREV_FRAME = platform::ICON_FA_CARET_LEFT;
 static constexpr const char* ICON_NEXT_FRAME = platform::ICON_FA_CARET_RIGHT;
 static constexpr const char* ICON_CURR_FRAME = platform::ICON_FA_FAST_FORWARD;
-static constexpr const char* ICON_RELOAD_SPEC = platform::ICON_FA_REFRESH;
 static constexpr const char* ICON_UNDO_SPEC = platform::ICON_FA_UNDO;
 static constexpr const char* ICON_REDO_SPEC = platform::ICON_FA_REPEAT;
 
@@ -1402,13 +1400,12 @@ void App::HelpGui() {
 }
 
 void App::ToolBarGui() {
+  ImGui::PushStyleVar(ImGuiStyleVar_CellPadding, ImVec2(0, 0));
   if (ImGui::BeginTable("##ToolBarTable", 2)) {
     platform::ScopedStyle style;
-    const ImColor red(220, 40, 40, 255);
-
-    const float scale = ImGui::GetWindowDpiScale();
-    const ImVec2 button_size(48.f * scale, 32.f * scale);
-    const ImVec2 play_button_size(80.f * scale, 32.f * scale);
+    style.Var(ImGuiStyleVar_ItemSpacing,
+              ImVec2(ImGui::GetStyle().ItemSpacing.x * 2.0f,
+                     ImGui::GetStyle().ItemSpacing.y));
 
     const float label_width = GetExpectedLabelWidth();
     const float copy_btn_width = ImGui::CalcTextSize(ICON_COPY_CAMERA).x +
@@ -1420,42 +1417,29 @@ void App::ToolBarGui() {
     const float right_width = label_width + sp + label_width + sp +
                               label_width + sp + copy_btn_width + sp +
                               theme_width;
-    const float separator_width = .2f * button_size.x;
+    const float separator_width = ImGui::GetFrameHeight() * .6f;
 
     ImGui::TableSetupColumn("", ImGuiTableColumnFlags_WidthStretch);
     ImGui::TableSetupColumn("", ImGuiTableColumnFlags_WidthFixed, right_width);
 
     ImGui::TableNextColumn();
+    ImGui::SetCursorPosX(ImGui::GetCursorPosX() + ImGui::GetStyle().WindowPadding.x);
 
-    // Combined (Unload, Reload) widget
+    const float btn_size = ImGui::GetFrameHeight();
+    const ImVec2 square_size(btn_size, btn_size);
+
+    // Reload button.
     {
       style.Var(ImGuiStyleVar_FrameRounding, 2.0f);
-
-      // Unload button.
-      {
-        const ImColor a = red;
-        const ImColor h(a.Value.x, a.Value.y, a.Value.z, a.Value.w * 0.6f);
-        style.Color(ImGuiCol_ButtonHovered, h);
-        style.Color(ImGuiCol_ButtonActive, a);
-
-        if (ImGui::Button(ICON_UNLOAD_MODEL, button_size)) {
-          InitEmptyModel();
-        }
-        ImGui::SetItemTooltip("%s", "Unload");
-        style.Reset();
-      }
-
-      // Reload button.
-      ImGui::SameLine(0, 0);
-      if (ImGui::Button(ICON_RELOAD_MODEL, button_size)) {
+      if (ImGui::Button(ICON_RELOAD_MODEL, square_size)) {
         RequestModelReload();
       }
       ImGui::SetItemTooltip("%s", "Reload");
     }
 
     // Reset button.
-    ImGui::SameLine(0, separator_width);
-    if (ImGui::Button(ICON_RESET_MODEL, button_size)) {
+    ImGui::SameLine(0, 0.5 * separator_width);
+    if (ImGui::Button(ICON_RESET_MODEL, square_size)) {
       ResetPhysics();
     }
     ImGui::SetItemTooltip("%s", "Reset");
@@ -1465,10 +1449,8 @@ void App::ToolBarGui() {
     platform::StepControlGui(model(), &step_control_, tmp_.speed_index);
 
     ImGui::TableNextColumn();
-    ImGui::SetCursorPosY(ImGui::GetCursorPosY() +
-                         (button_size.y - ImGui::GetFrameHeight()) * 0.5f);
 
-    if (ImGui::Button(ICON_COPY_CAMERA)) {
+    if (ImGui::Button(ICON_COPY_CAMERA, square_size)) {
       std::string camera_string = platform::CameraToString(data(), &camera_);
       platform::MaybeSaveToClipboard(camera_string);
     }
@@ -1488,13 +1470,14 @@ void App::ToolBarGui() {
 
     ImGui::SameLine();
     ImGui::SetNextItemWidth(GetExpectedLabelWidth());
-    if (platform::ThemeSelectGui(&ui_.theme)) {
+    if (platform::ThemeSelectGui(&ui_.theme, square_size)) {
       platform::SetupTheme(ui_.theme);
       ImGui::GetIO().WantSaveIniSettings = true;
     }
 
     ImGui::EndTable();
   }
+  ImGui::PopStyleVar();
 }
 
 void App::StatusBarGui() {