first commit

content.js

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+(function() {
+    // ボタン群を包むコンテナのID
+    const CONTAINER_ID = 'yt-swift-control-container';
+
+    function updateSpeedButtonStyle(btn, rate) {
+        if (rate === 2) {
+            btn.style.backgroundColor = "#ff0000";
+            btn.style.color = "#ffffff";
+            btn.style.fontWeight = "bold";
+            btn.innerHTML = '2.0x';
+        } else {
+            btn.style.backgroundColor = "rgba(255, 255, 255, 0.1)";
+            btn.style.color = "#ffffff";
+            btn.style.fontWeight = "normal";
+            btn.innerHTML = '1.0x';
+        }
+    }
+
+    function createControlBtn(text, title, onClick) {
+        const btn = document.createElement('button');
+        btn.className = 'ytp-button';
+        btn.innerHTML = text;
+        btn.title = title;
+        btn.style.width = 'auto';
+        btn.style.minWidth = '45px';
+        btn.style.height = '30px';
+        btn.style.marginTop = '8px';
+        btn.style.marginRight = '5px';
+        btn.style.borderRadius = '4px';
+        btn.style.fontSize = '12px';
+        btn.style.display = 'inline-flex';
+        btn.style.alignItems = 'center';
+        btn.style.justifyContent = 'center';
+        btn.style.transition = 'background-color 0.2s';
+        btn.style.verticalAlign = 'top';
+        btn.onclick = onClick;
+        return btn;
+    }
+
+    function injectControls() {
+        const controls = document.querySelector('.ytp-right-controls');
+        if (!controls || document.getElementById(CONTAINER_ID)) return;
+
+        const container = document.createElement('div');
+        container.id = CONTAINER_ID;
+        container.style.display = 'inline-flex';
+        container.style.marginRight = '10px';
+        container.style.verticalAlign = 'top';
+
+        const video = document.querySelector('video');
+
+        // 1. 10秒巻き戻しボタン
+        const prevBtn = createControlBtn('←10', '10秒巻き戻す', () => {
+            if (video) video.currentTime -= 10;
+        });
+
+        // 2. 速度切り替えボタン
+        const speedBtn = createControlBtn('1.0x', '再生速度を1x/2xで切り替え', () => {
+            if (video) {
+                const newRate = video.playbackRate === 1 ? 2 : 1;
+                video.playbackRate = newRate;
+                updateSpeedButtonStyle(speedBtn, newRate);
+            }
+        });
+        updateSpeedButtonStyle(speedBtn, video ? video.playbackRate : 1);
+
+        // 3. 10秒スキップボタン
+        const nextBtn = createControlBtn('10→', '10秒スキップ', () => {
+            if (video) video.currentTime += 10;
+        });
+
+        container.appendChild(prevBtn);
+        container.appendChild(speedBtn);
+        container.appendChild(nextBtn);
+        controls.prepend(container);
+    }
+
+    const observer = new MutationObserver(injectControls);
+    observer.observe(document.body, { childList: true, subtree: true });
+})();

manifest.json

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+{
+  "manifest_version": 3,
+  "name": "YT Swift Control",
+  "version": "1.1",
+  "description": "10秒スキップ、巻き戻し、再生速度の瞬時切り替えボタンを追加します",
+  "permissions": ["activeTab"],
+  "icons": {
+    "128": "icon.png"
+  },
+  "action": {
+    "default_icon": "icon.png"
+  },
+  "content_scripts": [
+    {
+      "matches": ["https://www.youtube.com/*"],
+      "js": ["content.js"],
+      "run_at": "document_idle"
+    }
+  ]
+}