Fix: Optimize Drawing.bBox calls to prevent unnecessary reflows in complex DOMs

[imbansalaniket]

Description

This PR addresses a performance issue caused by repeated calls to Drawing.bBox which appends and remove test node to the tester element in the DOM, which were leading to unnecessary layout reflows—especially in scenarios involving complex DOM structures or a large number of CSS variables. These frequent reflows resulted in noticeable performance degradation during initial chart rendering and interactions like resizing, panning, zooming, or updating plots.

Background

• Profiling revealed that frequent ``getBoundingClientRectcalls were forcing synchronous reflows inDrawing.bBox`.
• While this was negligible in simpler DOM trees, in real-world usage with thousands of CSS variables and deep nesting, these layout recalculations became expensive.
• This resulted in degraded runtime performance, particularly visible during initial rendering and chart interaction which require layout updates.

Changes Introduced

1. Width/height optimization:

   • Directly use getBBox() when only size metrics are needed (avoiding full Drawing.bBox).

2. Drawing.bBox logic

   • For non-text nodes: switched from getBoundingClientRect to getBBox.
   • For text nodes: retained getBoundingClientRect to ensure correct positioning. ( as getBBox fails to compute position for text node correctly. Raised here )

Impact

• Significant reduction in forced reflows during plot rendering/updates.
• Maintains accurate text positioning while optimizing other node calculations.
• No changes to public API or rendering output.

[alexcjohnson]

Thanks @imbansalaniket! There are two cases I'd be concerned about here, which are the original reasons we created Drawing.bBox in the first place:

1. the <div> we're plotting into isn't part of the DOM yet.
2. the <div> or its parents have CSS transforms applied.

I suspect that when we originally wrote this getBBox didn't have sufficient support or gave browser-dependent output, but it looks like at least the support has been good for years now. And perhaps its distinct characteristics vs getBoundingClientRect mean we don't need to worry about CSS transforms anymore?

Anyway if we can ensure these cases still work, and fix the various test failures (there are a bunch of image tests that fail, you can see them in the artifacts here: https://app.circleci.com/pipelines/github/plotly/plotly.js/12418/workflows/12a27ec0-ea1e-45f9-9d25-196d14e5e1b7/jobs/275741/artifacts - also looks like one jasmine test failed here: https://app.circleci.com/pipelines/github/plotly/plotly.js/12418/workflows/12a27ec0-ea1e-45f9-9d25-196d14e5e1b7/jobs/275729) then I'm all in favor of this change.

I guess the other thing to say is we really shouldn't need to do this at all for non-text nodes. I know there are a few cases where we currently do call Drawing.bBox on non-text nodes but in principle we can directly calculate what we need for these cases, which will always be better than asking the browser.