bug(cdk/scrolling): The append only mode assumes that scrolling starts at zero and therefore calculates renderedRange incorrectly

[philippspindler]

Reproduction

Steps to reproduce:

1. Create a virtual scroll container with appendOnly
2. Load the virtual scroll container and don't load it from the top
3. The renderedRange is wrongly calculated/set

Expected Behavior

A virtual scroll container can be loaded at any position without renderedRange being set incorrectly.

Actual Behavior

The renderedRange was set incorrectly once the virtual scroll container was loaded not at the top. I archived this behaviour via the following CSS setting.

[cdk-virtual-scroll-viewport] {
  overflow-anchor: none;
}

A possible solution

I was able to solve this issue by overriding the following code at virtual-scroll-viewport.ts#L292

if (this.appendOnly) {
-  range = {start: 0, end: Math.max(this._renderedRange.end, range.end)};
+  range = {start: Math.min(this._renderedRange.start, range.start), end: Math.max(this._renderedRange.end, range.end)};
}

I am not sure if I am missing something. For this reason, I have just opened an issue and no pull request for now.

Environment

• Angular: 12.2.5
• CDK/Material: 12.2.5
• Browser(s): Chrome
• Operating System (e.g. Windows, macOS, Ubuntu): macOS