Hover tooltip can be cached to a wrong location

[yamaton]

Description

When a mouse cursor is moving when a hover is triggered, the tooltip information can be stored to a wrong location. Here, I mean "trigger" by the initial appearance of the underline to a target token.

In this example, sin shows an incorrect tooltip that is actually from math. The data is cached so this behavior stays once it occurs.

Reproduce

[Optional but recommended] Go to "Advanced Setting Editor" → "Code hover" → "Throttler delay", and set the value to 500 or more to make the reproduction easy.

Point a token with a mouse cursor, and then point another token within the delay period. Timing of Ctrl key pressing does not matter, by the way.

Discussion

I found this problem when I was preparing a PR for #362. The root of this problem is that the throttler is not always in the same page as each caller. Specifically,
this.debounced_get_hover.invoke() does not always use the same this.virtual_position as it was assigned before the call if the throttler is invoked multiple times during a period.

https://github.com/jupyter-lsp/jupyterlab-lsp/blob/master/packages/jupyterlab-lsp/src/features/hover.ts#L446

I could get around this hover problem by checking if this.debounced_get_hover.invoke() referred to the same virtual position as the one in _updateUnderlineAndTooltip. Will send a PR later.