Adding null check before clearing mouseDownTimeout

[nzayatz14]

Hello,

I was recently doing some performance testing on a virtual table component I had written. With rows constantly unmounting & remounting, I wanted to make sure the scroll was still smooth with tooltips (rc-tooltips to be exact).

During my tests, I noticed that a significant amount of my table cells needed to clear a timeout upon unmounting of the row, this caused lag in my scrolling.

I found the call was made by rc-trigger in it's componentWillUnmount function. I added a simple null-check before the clearTimeout function and it significantly improved my scrolling performance.

I also noticed that the other timer, this.delayTimer, has its own function that handles this case -> this.clearDelayTimer() and was wondering why this.mouseDownTimeout did not have something similar.

Thanks!
Nick

[codecov]

Codecov Report

Merging #201 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #201   +/-   ##
=======================================
  Coverage   88.22%   88.22%           
=======================================
  Files          10       10           
  Lines         484      484           
  Branches      121      122    +1     
=======================================
  Hits          427      427           
  Misses         57       57           

Impacted Files	Coverage Δ
src/index.tsx	83.78% <100.00%> (ø)

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[shc023]

I was also running into perf issue caused by the same code when unmounting 10k+ rc-tooltip at the same time. I made a PR that does this and also clears the pointer itself after executing, which potentially gives even more performance gain:

#278