Vivaldi window state management on macOS has gotten better over time but still suffers from some annoying bugs.
If you quit Vivaldi (Command-Q) and relaunch, it remembers the window size reliably, including the maximized and fullscreen states.
If at least one Vivaldi window is open and another one is opened with Command-N, it will (almost always) open with the correct size.
However, if you close all Vivaldi windows and open a new one with Command-N, it consistently opens with the "default" (wrong) window size. Incidentally, quitting Vivaldi in this state and relaunching will create windows of the "correct" size again, so it hasn't completely lost the desired dimensions and positions.
Thanks for sharing your frustration; Vivaldi needs to be reminded that their browser's non-native quirks are annoying and driving away some Mac users. However, Vivaldi's UI is also more Mac-like in many ways than Firefox or Chrome, and if you continue using Vivaldi and sidestep its current quirks, its many other positive attributes will also shine through.
For me, it comes to the foreground if an open Vivaldi window is in the same "space"/desktop as the app that had the link (an email app for instance). But if there is no open Vivaldi window in that space/desktop, then Vivaldi does not take the focus and come to the foreground. I have to go into mission control and start hunting through my desktops to find it or double click the Vivaldi icon in the dock to get it to take me to the desktop that has a Vivaldi window.
The expected behavior would be either to push me to the desktop that has the Vivaldi window open. I wouldn't mind if it just opened a new window on that desktop, but that wouldn't match the behavior of Safari or most other browsers and Mac apps (but would actually be an improvement over them in my opinion).
@Ayespy I use Vivaldi on my Windows machine as well, and the delay is not present. This seems isolated to MacOS. I don't have another Mac handy to test either, to see if it is something isolated to my machine.
@alessandro.paladino: One of the reasons some Chrome extensions have no effect on Vivaldi is that the Vivaldi UI is unrelated to Chromium, and cannot be affected by the things which are designed to plug into or configure the Chromium UI. Special code has to be written to make UI modifications from Chrome extensions appear in the Vivaldi UI.
Although Vivaldi uses the Chromium and V8 ENGINES, it is not a Chrome knock-off browser as most Chromium-based browsers are.
Yeah, suggestions of switching to AppKit are probably nonstarters. Stuff like "Look Up" and Ctrl-Command-Space to bring up the Characters browser seem likely to be doable though; Chrome has those working.
As for spell-checking, I'm noticing that in both Vivaldi and Chrome on OSX, no matter which languages I have added in Language Settings (chrome://settings/languages or right-click on text, select "Language Settings"), even if I just have English set (you can multiple languages set), the red zigzag underline appears only under words that don't match any word in some multi-language (but not every-language) wordlist. (Some testing results of mine: Correct English, Portuguese, Russian, and Spanish words are not red-underlined. Hebrew letters it doesn't even try to spellcheck. For Aafrikans text, Vivaldi red-underlines a lot of words but not all, presumably recognizing some from a Dutch wordlist.)
Also, in both Vivaldi and Chrome on OSX, although there is a context menu option to add a word to the dictionary, there is no way to edit or remove such words once added. In Windows, chrome://settings/editDictionary lets you edit it, and in chrome://settings/languages/ there is an option at the bottom to enable/disable spell checking entirely as shown in screenshots at http://www.thewindowsclub.com/remove-words-chrome-spelling-dictionary - something missing from chrome://settings/languages/ on OSX. (Submitted as a bug, just now.)
I managed to get it to work on osx by clicking both trackpad keys at the same time. Funnily enough this doesn't work by clicking left and right mouse button, and therefore this probably won't help you if you are on a real mac. The only other option is using something like Karabiner, to assign the mousewheel click to a keyboard shortcut. Currently not functional on Sierra.
I also read about pressing ctrl and command and clicking to simulate the mousewheel click, but it doesn't work for me.