Allow mod-tap hold action on one shot layer

[kosorin]

Description

This proposal allows mod-tap hold action on one shot layer. One shot layer is immediately disabled if MT() is pressed and therefore mod-tap hold is not possible.

Example:

Following example currently produce a lower case character x.

 [L1] = LAYOUT(KC_X, OSL(L2))
 [L2] = LAYOUT(KC_A, LSFT_T(...))

• on layer 1
• OSL(L2) down
• OSL(L2) up
• on layer 2
• LSFT_T(...) down
• KC_A down
• KC_A up
• LSFT_T(...) up

With the proposed change the result is a upper case A and it will behave same as KC_LSFT.

I'm not aware if this covers all possible situations, but I've tested it for several months with no problems.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[drashna]

Also, could you add to the unit tests to ensure that this behavior is properly tested?

[kosorin]

I added a few unit tests and fixed a small bug that I discovered thanks to the tests. I'm not sure that's enough. It certainly doesn't test all permutations of all Tap-Hold options.

[drashna]

I added a few unit tests and fixed a small bug that I discovered thanks to the tests. I'm not sure that's enough. It certainly doesn't test all permutations of all Tap-Hold options.

Well, having them at all is a huge thing. And they've already proven when even basic tests are definitely better than nothing, here. :)

At the very least, it helps with confidence in the code. And more robust testing can be added later.

[KarlK90]

Thanks for fixing bugs and adding unit-tests in the progress! This is much appreciated. The changes LGTM.