Add keepMounted option and fix screen glitch

[pikaju]

Adds option that keeps the Modal mounted even when it is hidden. This is useful for keeping the DOM state inside the Modal as well as for SEO purposes.

Fixes #233 and #495

[pradel]

Thanks for the pr! Could you please add an example in the docs so I can try it?

[pradel]

can you please explain why this is needed?

[pikaju]

This fixes a separate issue #495. I copied the solution from one of the people in that thread. To be honest, I'm not sure why it helps, but it does.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.16 🎉

Comparison is base (16aad65) 95.28% compared to head (d2c9bb4) 95.45%.

❗ Current head d2c9bb4 differs from pull request most recent head 11fd42a. Consider uploading reports for the commit 11fd42a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
+ Coverage   95.28%   95.45%   +0.16%     
==========================================
  Files           6        6              
  Lines         191      198       +7     
  Branches       69       74       +5     
==========================================
+ Hits          182      189       +7     
  Misses          9        9              

Impacted Files	Coverage Δ
react-responsive-modal/src/index.tsx	100.00% <100.00%> (ø)

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[pikaju]

Hey @pradel, thanks for the response. I added an example and docs in the Next project.

I also changed how modals are centered, from the "inline" behavior to flex-box centering. I did this because there was an issue when the modal had little space (e.g. on a mobile device) and the pseudo-element you had before was wrapped to a new line. This implementation doesn't do that.

[akoll]

Found a small typo:

Suggested change

	By setting the `keepMounted` property to `true`, you can specify that the modal should always be mounted to the DOM, even when hidden it is hidden.
	By setting the `keepMounted` property to `true`, you can specify that the modal should always be mounted to the DOM, even when it is hidden.

[pradel]

@pikaju could you please split the pr into two parts?

• the keepMounted can be released as a minor version first
• then the change you made to the CSS for the mobile issue, this one will need to be released as a breaking change as it may conflict with the CSS used by users of this package

[pikaju]

@pradel Sorry, but that doesn't seem worth the effort, I suggest just releasing a new major version