Windows 10's taskbar is an almost unchanged continuation of the design Microsoft introduced with Windows 7. By default, open windows are combined into a single icon representing their parent app.
If you've got just one window of an app open, you can quickly switch back to it by clicking its taskbar icon. When more than one window is open, Windows will instead display you the thumbnail strip letting you pick the window you want. If you frequently switch between apps with lots of open windows, this two-click procedure might become a little tedious.
Using a registry hack, it's possible to modify the behaviour here. We'll make it so that clicking a taskbar icon always opens the last used window of that app. You'll still be able to see and switch between all the open windows by hovering over the icon to display the thumbnail strip.
As always, you edit the registry at your own risk – this setting isn't officially supported by Microsoft, and might stop working in the future. This technique should work on every version of Windows, from 7 onwards, but we'll be focusing on Windows 10.
Open the Registry Editor by searching for "regedit" in the Start menu. You'll need to be logged in as an administrator. In the window that appears, use the tree view on the left to navigate to the following key:
If you're using a recent version of Windows 10, the Registry Editor has an address bar at the top of the window. You can paste the key in here to quickly navigate to your destination.
In the pane on the right, be careful not to edit any of the registry values which are displayed. Right-click the pane and select New > DWORD (32-bit) Value. Type "LastActiveClick" as the value for the new key.
Now, double click the key you just selected to open the value editor. Replace the value with "1" and click OK. You'll need to logout and back in again before the change takes effect. Open a few apps with multiple windows each, and try clicking their taskbar icons. You'll see you immediately switch to the last open window, without displaying the thumbnail strip.
Finally, it's worth noting this behaviour is built into Windows, although in a far-from-obvious way. Holding the control (Ctrl) key while clicking on a taskbar icon will always open the last active window, no registry hack required. If you only occasionally need this behaviour, holding the Ctrl key might be the best approach for you.