Update logic for dealing with file dialogs.

This in preparation for an upcoming change that uses native file dialogs instead
of our very basic ImGui file dialog.

PiperOrigin-RevId: 861671041
Change-Id: I9b00c05efdda5e37ffe15048a2ccf66f6eca89d9

src/experimental/platform/file_dialog.cc

@@ -0,0 +1,67 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "experimental/platform/file_dialog.h"
+
+#include <cstring>
+
+#include <span>
+#include <string>
+#include <string_view>
+
+#include <imgui.h>
+
+namespace mujoco::platform {
+
+static DialogResult ImGui_FileDialog(std::string_view path) {
+  static constexpr int kBufferSize = 1024;
+  char buf[kBufferSize];
+  std::strncpy(buf, path.data(), sizeof(buf));
+  buf[sizeof(buf) - 1] = '\0';
+
+  DialogResult::Status status = DialogResult::kPending;
+
+  ImGui::Text("Filename");
+  ImGui::SameLine();
+  ImGui::SetNextItemWidth(600);
+  if (ImGui::InputText("##Filename", buf, kBufferSize, ImGuiInputTextFlags_EnterReturnsTrue)) {
+    status = DialogResult::kAccepted;
+  }
+  if (ImGui::Button("OK", ImVec2(120, 0)) || ImGui::IsKeyChordPressed(ImGuiKey_Enter)) {
+    status = DialogResult::kAccepted;
+  }
+  ImGui::SetItemDefaultFocus();
+  ImGui::SameLine();
+  if (ImGui::Button("Cancel", ImVec2(120, 0)) || ImGui::IsKeyChordPressed(ImGuiKey_Escape)) {
+    status = DialogResult::kCancelled;
+  }
+
+  return DialogResult{.status = status, .path = std::string(buf)};
+}
+
+DialogResult OpenFileDialog(std::string_view path,
+                            std::span<std::string_view> filters) {
+  return ImGui_FileDialog(path);
+}
+
+DialogResult SaveFileDialog(std::string_view path,
+                            std::span<std::string_view> filters) {
+  return ImGui_FileDialog(path);
+}
+
+DialogResult SelectPathDialog(std::string_view path) {
+  return ImGui_FileDialog(path);
+}
+
+}  // namespace mujoco::platform