Index: gc_lang/fr/webext/content_scripts/init.js
==================================================================
--- gc_lang/fr/webext/content_scripts/init.js
+++ gc_lang/fr/webext/content_scripts/init.js
@@ -127,11 +127,11 @@
         }
     },
 
     createMessageBox: function () {
         if (this.oMessageBox === null) {
-            this.oMessageBox = new GrammalecteMessageBox("grammalecte_message_box", "Grammalecte", 400, 250);
+            this.oMessageBox = new GrammalecteMessageBox("grammalecte_message_box", "Grammalecte");
             this.oMessageBox.insertIntoPage();
         }
     },
 
     startGCPanel: function (xNode=null) {
@@ -156,11 +156,11 @@
     },
 
     showMessage: function (sMessage) {
         this.createMessageBox();
         this.oMessageBox.show();
-        this.oMessageBox.addMessage(sMessage);
+        this.oMessageBox.setMessage(sMessage);
     },
 
     getPageText: function () {
         let sPageText = document.body.innerText;
         let nPos = sPageText.indexOf("__grammalecte_panel__");

Index: gc_lang/fr/webext/content_scripts/message_box.css
==================================================================
--- gc_lang/fr/webext/content_scripts/message_box.css
+++ gc_lang/fr/webext/content_scripts/message_box.css
@@ -2,19 +2,25 @@
     CSS
     Message box for Grammalecte
 */
 
 .grammalecte_message_box {
+    position: fixed;
     padding: 0;
     margin: 0;
-    position: fixed;
+    top: 50%;
+    left: 50%;
+    width: 400px;
+    margin-left: -200px;
+    margin-left: -100px;
+    min-height: 100px;
     box-sizing: content-box;
-    z-index: 2147483641; /* maximum is 2147483647: https://stackoverflow.com/questions/491052/minimum-and-maximum-value-of-z-index */
-    border: 2px solid hsl(210, 10%, 50%);
+    z-index: 2147483647; /* maximum is 2147483647: https://stackoverflow.com/questions/491052/minimum-and-maximum-value-of-z-index */
+    border: 2px solid hsl(210, 50%, 50%);
     border-radius: 10px 10px 10px 10px;
-    background-color: hsl(210, 0%, 100%);
-    color: hsl(0, 0%, 0%);
+    background-color: hsl(210, 50%, 50%);
+    color: hsl(0, 10%, 92%);
     font-family: "Trebuchet MS", "Fira Sans", "Liberation Sans", sans-serif;
     box-shadow: 0 0 2px 1px hsla(210, 50%, 50%, .5);
     line-height: normal;
     text-shadow: none;
     text-decoration: none;
@@ -26,14 +32,14 @@
 }
 
 .grammalecte_message_box_bar {
     position: sticky;
     width: 100%;
-    background-color: hsl(210, 0%, 90%);
+    background-color: hsl(210, 50%, 50%);
     border-radius: 10px 10px 0 0;
-    border-bottom: 1px solid hsl(210, 10%, 80%);
-    color: hsl(210, 10%, 4%);
+    border-bottom: 1px solid hsl(210, 50%, 47%);
+    color: hsl(0, 10%, 92%);
     font-size: 20px;
 }
 .grammalecte_message_box_title {
     padding: 10px 20px;
 }
@@ -43,23 +49,14 @@
 }
 .grammalecte_message_box_invisible_marker {
     position: absolute;
     /*visibility: hidden;*/
     font-size: 6px;
-    color: hsl(210, 0%, 90%); /* same color than panel_bar background */
+    color: hsl(210, 50%, 50%); /* same color than panel_bar background */
 }
 
 .grammalecte_message_box_content {
-    position: absolute;
-    min-width: 100%;
     height: calc(100% - 55px); /* panel height - title_bar */
-    overflow: auto;
-}
-
-.grammalecte_message_box_message {
-    margin: 10px;
-    padding: 10px;
-    border-radius: 5px;
-    background-color: hsl(0, 50%, 40%);
+    padding: 20px;
     color: hsl(0, 50%, 96%);
-    font-size: 20px;
+    font-size: 18px;
 }

Index: gc_lang/fr/webext/content_scripts/message_box.js
==================================================================
--- gc_lang/fr/webext/content_scripts/message_box.js
+++ gc_lang/fr/webext/content_scripts/message_box.js
@@ -4,18 +4,15 @@
 "use strict";
 
 
 class GrammalecteMessageBox {
 
-    constructor (sId, sTitle, nWidth, nHeight) {
+    constructor (sId, sTitle) {
         this.sId = sId;
-        this.nWidth = nWidth;
-        this.nHeight = nHeight;
         this.xMessageBoxBar = oGrammalecte.createNode("div", {className: "grammalecte_message_box_bar"});
         this.xMessageBoxContent = oGrammalecte.createNode("div", {className: "grammalecte_message_box_content"});
         this.xMessageBox = this._createPanel(sTitle);
-        this.center();
     }
 
     _createPanel (sTitle) {
         try {
             let xMessageBox = oGrammalecte.createNode("div", {id: this.sId, className: "grammalecte_message_box"});
@@ -24,11 +21,10 @@
             let xTitle = oGrammalecte.createNode("div", {className: "grammalecte_panel_title"});
             xTitle.appendChild(this._createLogo());
             xTitle.appendChild(oGrammalecte.createNode("div", {className: "grammalecte_message_box_label", textContent: sTitle}));
             this.xMessageBoxBar.appendChild(xTitle);
             xMessageBox.appendChild(this.xMessageBoxBar);
-
             xMessageBox.appendChild(this.xMessageBoxContent);
             return xMessageBox;
         }
         catch (e) {
             showError(e);
@@ -64,19 +60,13 @@
     hide () {
         this.xMessageBox.style.display = "none";
         this.clear();
     }
 
-    addMessage (sMessage) {
-    	this.xMessageBoxContent.appendChild(oGrammalecte.createNode("div", {className: "grammalecte_message_box_message", textContent: sMessage}));
+    setMessage (sMessage) {
+        this.xMessageBoxContent.textContent = sMessage;
     }
 
     clear () {
-        while (this.xMessageBoxContent.firstChild) {
-            this.xMessageBoxContent.removeChild(this.xMessageBoxContent.firstChild);
-        }
-    }
-
-    center () {
-        this.xMessageBox.style = `top: 50%; left: 50%; width: ${this.nWidth}px; height: ${this.nHeight}px; margin-top: -${this.nHeight/2}px; margin-left: -${this.nWidth/2}px;`;
+        this.xMessageBoxContent.textContent = "";
     }
 }