===== Opera takes too long to load if not used recently (SOLVED) =====
This issue was encountered in Ubuntu 20.04 LTS, using GNOME.

Opening Opera takes 30-60 seconds, during which time the system gives no indication that it is attempting to open the application. After being opened, it can be closed and opened again reasonably quickly. 

  * There are error messages when opening Opera via the terminal:<code>
(opera:231402): Gtk-WARNING **: 14:10:24.091: Theme parsing error: gtk.css:1555:23: 'font-feature-settings' is not a valid property name
(opera:231402): Gtk-WARNING **: 14:10:24.094: Theme parsing error: gtk.css:3591:25: 'font-feature-settings' is not a valid property name
(opera:231402): Gtk-WARNING **: 14:10:24.094: Theme parsing error: gtk.css:4053:23: 'font-feature-settings' is not a valid property name
[231402:231402:0409/141108.748868:ERROR:CONSOLE(19)] "Uncaught (in promise) [object Object]", source: chrome://op-resources/modules/services/history.js (19)
[231402:231402:0409/141108.748928:ERROR:CONSOLE(19)] "Uncaught (in promise) [object Object]", source: chrome://op-resources/modules/services/history.js (19)
[231402:231814:0409/141114.437753:ERROR:udev_watcher.cc(97)] Failed to begin udev enumeration.
</code>
  * If opera has been closed recently, it opens much more quickly. There are still errors in the terminal, but they are different:<code>(opera:235537): Gtk-WARNING **: 14:30:46.047: Theme parsing error: gtk.css:1555:23: 'font-feature-settings' is not a valid property name
(opera:235537): Gtk-WARNING **: 14:30:46.050: Theme parsing error: gtk.css:3591:25: 'font-feature-settings' is not a valid property name
(opera:235537): Gtk-WARNING **: 14:30:46.051: Theme parsing error: gtk.css:4053:23: 'font-feature-settings' is not a valid property name
[235537:235638:0409/143054.462332:ERROR:udev_watcher.cc(97)] Failed to begin udev enumeration.
</code>
  * I have noticed that many applications installed via the Ubuntu Application Manage ("snap?") develop peculiar behaviors, which are fixed by uninstalling them, and reinstalling them via ''get-apt install'' commands in the terminal. I will attempt this soon.
  * I uninstalled Opera and attempted to reinstall it with the apt package manager. There is no apt package for Opera, so I reinstalled it with snap via command line. The problem persists.
  * Based on the advice [[https://forums.opera.com/topic/38293/solved-opera-66-0-3515-72-1-crashing-without-any-extension-or-new-installation/60?lang=en-US | here]], I attempted to launch Opera using ''opera --no-sandbox''. This didn't work, but I got a different set of error messages:<code>(opera:7904): Gtk-WARNING **: 15:24:25.130: Theme parsing error: gtk.css:1555:23: 'font-feature-settings' is not a valid property name

(opera:7904): Gtk-WARNING **: 15:24:25.133: Theme parsing error: gtk.css:3591:25: 'font-feature-settings' is not a valid property name

(opera:7904): Gtk-WARNING **: 15:24:25.133: Theme parsing error: gtk.css:4053:23: 'font-feature-settings' is not a valid property name
[7904:7904:0410/152453.809696:ERROR:named_platform_channel_posix.cc(145)] connect /home/beloch/snap/opera/118/.config/opera/oauc_pipe_r: Connection refused (111)
[7904:7904:0410/152503.812667:ERROR:named_platform_channel_posix.cc(145)] connect /home/beloch/snap/opera/118/.config/opera/oauc_pipe_r: Connection refused (111)
[7904:7904:0410/152504.814020:ERROR:named_platform_channel_posix.cc(145)] connect /home/beloch/snap/opera/118/.config/opera/oauc_pipe_r: Connection refused (111)</code>
  * Based on the AskUbuntu thread [[https://askubuntu.com/questions/1328843/very-slow-startup-of-some-browsers | "Very slow startup of some browsers,"]], I investigated appmenu-gtk2-module & appmenu-gtk3-module. Both are fairly small and seem to modify the system very little, so I installed them. This did not fix my issue. I also ran ''lspci -vnn | grep "VGA"'' in hopes I might see some relevant error, but the output seems benign to me. <code>0a:00.0 VGA compatible controller [0300]: NVIDIA Corporation TU116 [GeForce GTX 1660] [10de:2184] (rev a1) (prog-if 00 [VGA controller])
</code>
  * After installing GTK I foolishly did not open Opera using the command line, so I was not able to inspect any error messages that may have appeared during its slow start. //However//, opening Opera again produces two "sets" of errors, with the second being displayed after a fresh input prompt appears. This is new behavior. The second error message is also new. (It is very disorienting that my problem is consistent, but my error messages are not!)<code>beloch@workhorse:~$ opera

(opera:68236): Gtk-WARNING **: 16:02:08.016: Theme parsing error: gtk.css:1555:23: 'font-feature-settings' is not a valid property name

(opera:68236): Gtk-WARNING **: 16:02:08.019: Theme parsing error: gtk.css:3591:25: 'font-feature-settings' is not a valid property name

(opera:68236): Gtk-WARNING **: 16:02:08.019: Theme parsing error: gtk.css:4053:23: 'font-feature-settings' is not a valid property name
beloch@workhorse:~$ [68337:68337:0413/160208.083377:ERROR:broker_posix.cc(43)] Invalid node channel message
[68335:68335:0100/000000.179966:ERROR:broker_posix.cc(43)] Invalid node channel message
</code>
  * Inputting ''lsmod | egrep 'nouveau|nvidia''' in the terminal outputs:<code>nvidia_uvm           1011712  0
nvidia_drm             49152  4
nvidia_modeset       1183744  8 nvidia_drm
nvidia              19746816  370 nvidia_uvm,nvidia_modeset
drm_kms_helper        217088  1 nvidia_drm
drm                   552960  7 drm_kms_helper,nvidia_drm
i2c_nvidia_gpu         16384  0
</code>

=== SOLUTION: ===
As I postulated earlier, the issue was solved by getting rid of SNAP. A friend was able to hold my hand through the process. 

  - I started by uninstalling snap's Opera with ''sudo snap remove opera'' in the terminal.
  - On the [[https://www.opera.com/download | Opera website]] I downloaded their .deb install file. (currently opera-stable_75.0.3969.171_amd64.deb, but presumably this is updated regularly.)
  - In the terminal, I installed this. I am told it is essential to indicate the full file path for this process (no ~/). Additionally, the 'tab' key was helpful in automatically filling out the specific filename. ''sudo apt install /home/beloch/Downloads/opera-stable_75.0.3969.171_amd64.deb''

This completely resolved the issue. Opera now opens immediately, without any errors at all being displayed in the command line.