2070937 - A page's scroll handler runs while the debugger is paused

Status: NEW

(DevTools :: Debugger, defect)

Description

[Florian Quèze [:florian]]

With the debugger paused at a debugger statement, scrolling the page runs the page's own onscroll listener to completion instead of deferring it to the resume. Reproduced locally on a debug build with browser_dbg-scroll-run-to-completion.js un-skipped.

EventLoop.preEnter() sets document.pausedByDevTools, and since Bug 1967931 Document::IsRenderingSuppressed() deliberately returns false in that case, so that highlighters and the inspector still work on a paused page. nsRefreshDriver::Tick therefore keeps running every rendering phase for the document, including RenderingPhase::ScrollSteps, which dispatches queued scroll events to page listeners.

A fix would filter the script-running phases out of RunRenderingPhase for a document with PausedByDevTools(), and re-schedule them on resume so the events are deferred rather than dropped. Whether a resumed page gets one catch-up event is Bug 1968387's open question.

Comment 1

[BugBot [:suhaib / :marco]]

:emilio, since you are the author of the regressor, bug 1967931, could you take a look? Also, could you set the severity field?

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Comment 2

[Emilio Cobos Álvarez [:emilio]]

This is expected, see also this line in the regressing bug.

We can add a knob, but my understanding is that people wanted debugger pauses to still do things like running CSS animations etc. That also runs script.

In general, I'm opposed to whack-a-mole'ing rendering phases in and out of this behavior. Either the rendering is suppressed (with all phases not running) or it isn't.

Comment 3

[BugBot [:suhaib / :marco]]

Set release status flags based on info from the regressing bug 1967931