What is the work of 'Stack.ts' in 'bokehjs'?

I am working on this issue : [BUG] vbar_stack with DataFrame as source ignores entire row if first data column is NaN #9568

As advised by @Bryan in https://github.com/bokeh/bokeh/issues/9568#issuecomment-583870781 I checked out Stack.ts but I couldn’t understand what it’s supposed to do.

Searching for Stack in ./bokehjs/src reveals that it hasn’t been imported anywhere.

$ grep -rnw './bokehjs/src' -e 'Stack'

./bokehjs/src/lib/models/expressions/index.ts:2:export {Stack}      from "./stack"
./bokehjs/src/lib/models/expressions/stack.ts:6:export namespace Stack {
./bokehjs/src/lib/models/expressions/stack.ts:14:export interface Stack extends Stack.Attrs {}
./bokehjs/src/lib/models/expressions/stack.ts:16:export class Stack extends Expression {
./bokehjs/src/lib/models/expressions/stack.ts:17:	properties: Stack.Props;
./bokehjs/src/lib/models/expressions/stack.ts:19:	constructor(attrs?: Partial<Stack.Attrs>) {
./bokehjs/src/lib/models/expressions/stack.ts:24:		this.define<Stack.Props>({
./bokehjs/src/lib/core/layout/alignments.ts:6:export abstract class Stack extends Layoutable {
./bokehjs/src/lib/core/layout/alignments.ts:10:export class HStack extends Stack {
./bokehjs/src/lib/core/layout/alignments.ts:38:export class VStack extends Stack {

So, what is the work of ‘Stack.ts’ in ‘bokehjs’ ?

You didn’t find anything because you’re looking in the bokehjs directory.
In the case of the issue that you linked, BokehJS models are created based on Python models. Look up the usages of Stack in the Python code.