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When I press commandH, the current window is hidden. I'd like to disable this shortcut, because I'm using it in another application.

I opened System Preferences -> Hardware -> Keyboard -> Keyboard Shortcuts -> Application Shortcuts -> All Applications

I tried adding "No Action" as MenuTitle and commandH as KeyboardShortcut
(see screenshot below).

But this doesn't work.

What am I doing wrong?

Edit: What solved my problem was assigning shortcut commandOptionShiftH to actions like "Hide Eclipse". Since I never press this key combination, this action never gets executed.

another way to do this is to use BetterTouchTool and define a global keyboard shortcut which maps Command-H to no-action.

No app restart required.

See: BetterTouchTool

This tool is also useful to restore the original behavior of the red/orange/green window decorations in OSX

Adding an answer based on the Edit: above as it took me a while to figure this out.

If you want to disable the ⌘-h shortcut you have to do it per-app.

First open the app, and confirm from the preferences what text option the shortcut is mapped to.

Using Kitty.app as an example, for me it's Hide kitty.

Then you open System Preferences and go to Keyboard -> Shortcuts -> App Shortcuts and then click +.

Limit it to the relevant app, enter Hide <yourappname> (e.g. Hide kitty in my case), and pick an obscure key combination. I use the section symbol because it's not in my default layout so I won't type it by mistake.

Now when you click on the Application's menu, you should see your new shortcut has been set, and ⌘-h will no longer hide that application.

Unfortunately there doesn't seem to be a way to do this for all apps.

See also this Apple Support Page

The (very non-ideal, but functional) way that I solved this problem is to use Alfred to define a global hotkey mapping CommandH to nothing. You could use any app capable of defining global hotkeys to do this (ex. BetterTouchTool or KeyboardMaestro).

Here's the Alfred workflow that I used:

Adding yet another option:

If you use the Karabiner app (extremely useful in its own right -- for example, you can add VIM-style navigation with Ctrl+h/j/k/l, or map the Escape key to Caps Lock), there is a "Complex Modification" that disables command-h window hiding. Install the app and see the listing: Prevent unintended command-h hide window on this page.

Overriding keyboard shortcuts Via macOS Terminal

In this example, I change the "Open Viewer" menu option in Apple Photos to "Space", like it used to be pre-2017:

defaults write com.apple.Photos NSUserKeyEquivalents -dict-add "Open Viewer" -string " "

Reset to defaults:

defaults delete com.apple.Photos NSUserKeyEquivalents

Change the global shortcut for "Preferences" to "⌘ + p" (the "-g" option means "Global", and "@" translates to "⌘"):

defaults write -g NSUserKeyEquivalents -dict-add "Preferences" -string "@\p"

A word of warning though:
Do not paste this into your Terminal window if you don't feel comfortable with altering the default settings of macOS apps, or if you don't fully understand what it means.