Steps to Reproduce:
Pressing ALT highlights the mnemomics in the application menu allowing one to click any of them to open the menu (E.g. ALT-H to open the Help menu and ALT-S for the selection menu). This does not work when GitLens is disabled since the extension seems to hijack some of the combinations.

Please open the GitLens settings (run GitLens: Open Settings from the command palette) and jump to Keyboard Shortcuts and change it to either the chorded set or custom set. The alternate set which used to be the default, can both conflict with mnemonics and entering characters in different keyboard layouts.
While the chorded set is the new default, I didn't want to change the settings of existing installs.
Hope that helps.
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Most helpful comment
Please open the GitLens settings (run GitLens: Open Settings from the command palette) and jump to Keyboard Shortcuts and change it to either the
chorded setorcustom set. Thealternate setwhich used to be the default, can both conflict with mnemonics and entering characters in different keyboard layouts.While the
chorded setis the new default, I didn't want to change the settings of existing installs.Hope that helps.