[UI] Ensure prompts are always centered relative to the parent window in multi-monitors setups.

src/ui/Prompts.js

@@ -37,12 +37,13 @@ var Prompts = (() => {
     async open(data) {
       promptData = data;
       this.close();
-      let {width, height} = data.features;
+      let {width, height, left, top, parent = await browser.windows.getCurrent() } = data.features;
       let options = {
         url: browser.runtime.getURL("ui/prompt.html"),
-        type: "panel",
+        type: "popup",
         width,
         height,
+        focused: false, // initially in the background while sizing
       };
       if (UA.isMozilla) {
         options.allowScriptsToClose = true;
@@ -52,15 +53,31 @@ var Prompts = (() => {
         this.currentTab = await browser.tabs.create({url: options.url});
         return;
       }
-      this.currentWindow = await browser.windows.create(options);
-      // work around for https://bugzilla.mozilla.org/show_bug.cgi?id=1330882
-      let {left, top, width: cw, height: ch} = this.currentWindow;
-      if (width && height && cw !== width || ch !== height) {
-        left += Math.round((cw - width) / 2);
-        top += Math.round((ch - height) / 2);
-        for (let attempts = 2; attempts-- > 0;) // top gets set only 2nd time, moz bug?
-          await browser.windows.update(this.currentWindow.id,
-              {left, top, width, height});
+
+      let popup = this.currentWindow = await browser.windows.create(options);
+
+      if (parent) {
+        // center to the given parent window (default last focused browser tab)
+        if (left === undefined) left = Math.round(parent.left + (parent.width - popup.width) / 2);
+        if (top === undefined) top = Math.round(parent.top + (parent.height - popup.height) / 2);
+      } else {
+        // features.parent explicitly nulled: use given left & top or default to auto-centering on main screen
+        if (left === undefined) ({left} = popup);
+        if (top === undefined) ({top} = popup);
+      }
+
+      // work around for letterboxing changes (https://bugzilla.mozilla.org/show_bug.cgi?id=1330882)
+      let {width: popupWidth, height: popupHeight} = popup;
+      if (width && height && (popupWidth !== width || popupHeight !== height)) {
+        left += Math.round((popupWidth - width) / 2);
+        top += Math.round((popupHeight - height) / 2);
+      }
+
+      for (let attempts = 2; attempts-- > 0;) // position gets set only 2nd time, moz bug?
+        await browser.windows.update(popup.id,
+          {left, top, width, height, focused: false});
+      if (parent) {
+        await browser.windows.update(parent.id, {focused: true});
       }
     }
     async close() {

src/ui/prompt.js

@@ -128,7 +128,7 @@
     }
   });
 
-  let resize = async e => {
+  let fitHeight = async e => {
     if (!("windows" in browser)) {
       // tabbed (mobile?) - ensure buttons are visible
       document.querySelector("#buttons").scrollIntoView();
@@ -136,15 +136,16 @@
     }
     let win = await browser.windows.getCurrent();
     let delta = document.documentElement.offsetHeight - window.innerHeight;
-    let geometry = {
-      width: win.width, height: win.height + delta,
-      left: win.left, top: win.top - Math.round(delta / 2)
-    };
-    for (let j = 2; j-- > 0;) await browser.windows.update(win.id, geometry);
+    await browser.windows.update(win.id, {
+      height: win.height + delta,
+      top: win.top + Math.round(delta / 2),
+      focused: false
+    });
+    await browser.windows.update(win.id, {focused: true});
   }
   if (document.readyState === "complete") {
-    resize();
+    fitHeight();
   } else {
-    window.addEventListener("load", resize);
+    window.addEventListener("load", fitHeight);
   }
 })();