Slow to create many Watch dropdown expressions

[ariffjeff]

Issue Type: Bug

The Watch dropdown in the Debug tab becomes slow to respond to user input when there are many expressions being watched in it.

To reproduce (I'm using node js for debugging):
New js file...
Create lots of variables (I don't know if the size of the information in the variables matters or not so I just added a decently sized array):
a=b=c=d=e=f=g=h=i=j=k=l=m=n=o=p=q=r=s=t=u=v=w=x=y=z= [12,253,5,457,32,3647,48,56,679,567,45]

Then in the Watch dropdown, create a separate expression for every variable.
i.e.
a
b
c
d
...

Since I've done the whole alphabet you should have 26 individual Watch expressions. You should notice that the dropdown starts to take longer and longer to respond as you add more and more expressions.

VS Code version: Code - Insiders 1.42.0-insider (629a08e, 2020-01-07T05:28:24.258Z)
OS version: Windows_NT x64 10.0.18362

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz (12 x 3192)
GPU Status	2d_canvas: enabled flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled metal: disabled_off multiple_raster_threads: enabled_on oop_rasterization: disabled_off protected_video_decode: unavailable_off rasterization: enabled skia_renderer: disabled_off surface_control: disabled_off surface_synchronization: enabled_on video_decode: enabled viz_display_compositor: enabled_on viz_hit_test_surface_layer: disabled_off webgl: enabled webgl2: enabled
Load (avg)	undefined
Memory (System)	7.89GB (1.54GB free)
Process Argv
Screen Reader	no
VM	0%

Extensions: none

[isidorn]

Yes I can reproduce this.
And this behavior is not new. The issue is that when a new watch expression is added we have to re-evaluate each watch expression.
I have pushed some improvments that we do not re-evalaute each expression 3 times, but onle 1. So it should be 3 times faster.
Please note that apart from this we are seeing some slowdown in the processing of messages caputred with this issue microsoft/vscode-js-debug#206
Once we fix that, this one might get even better

fyi @connor4312

[ariffjeff]

Sounds good, thanks for explaining!