How to make Ubuntu 18.04 (GNOME) workspace prohibit alt-tab to windows on other workspaces
To ensure that the default task switcher of Gnome Shell (Ubuntu 17.10 and higher) only shows applications of the current workspace, change a setting using the following command:
gsettings set org.gnome.shell.app-switcher current-workspace-only true
You can also change this setting with dconf-editor, a graphical tool that allows inspecting and changing a range of settings of any application that uses the gsettings framework.
To reset to the default setting:
gsettings reset org.gnome.shell.app-switcher current-workspace-only
Alternate Tab extension : https://extensions.gnome.org/extension/15/alternatetab/
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Workspace Isolated Dash extension : https://extensions.gnome.org/extension/887/workspace-isolated-dash/ (Isn't work by default with Dash to panel!) or use Dash to panel https://extensions.gnome.org/extension/1160/dash-to-panel/ Settings -> Behavior -> Isolate Workspaces