Microsoft: We want to fix Chromium's 'janky' mouse scrolling with a dose of Edge

That's not how you handle a mouse scroll, Microsoft tells Chromium developers.

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Microsoft's browser developers think the Chromium approach to mouse scrolling is so bad that it's really a bug and that it can be solved with some smarts from Edge.     

Microsoft has shared its thoughts on Chromium's mouse behavior in a proposal to bring the Microsoft Edge browser's "smooth scrolling" functionality to Chromium that would fix the "janky" movements when scrolling on the scrollbar using Google's existing methods.   

The proposal comes as Microsoft gears up to release its first preview of the newly-minted Chromium-based version of Edge, which swaps out the EdgeHTML rendering engine for Chromium's Blink. 

As part of the shift towards Chromium, Microsoft has said it will contribute differentiating features back to the Chromium open-source project, including browser-accessibility features, as well as optimizations for touch and Chromium for Arm.  

But Microsoft also has its eye on how Chromium interacts with the mouse, an input device that Microsoft has been working on for decades, bringing the world the IntelliMouse in 1996 and the Microsoft Mouse three years earlier. 

As reported by Thurrott.com, Microsoft's proposal aims to address the high latency when people use a mouse with Chrome to slide a page up and down from the scrollbar.   

"Scrollbar scrolling using the mouse happens on the main thread in Chromium. If the main thread is busy, scrolling by clicking on the scrollbar will appear to be janky as the events keep getting queued up [and] wait for the main thread to free up," writes a Microsoft engineer, pointing to a Microsoft Edge page on GitHub that explains how Microsoft thinks a browser should handle mouse events.     

"We observe on average scrollbar drags have [over double the] latency in Chromium as compared to EdgeHTML today, worse on particularly busy sites. We attribute this gap primarily to EdgeHTML's feature for off-thread scrollbar drags.  By avoiding the main thread in Chromium as well, we believe we can bring the performance of scrollbar drags more in line with what we observe in EdgeHTML."   

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Microsoft has also filed an issue on Chromium's bug-report site, including a file that shows how Chromium handles scrollbar mouse controls. Microsoft notes that the browser currently handles scrolling in the main thread and that to avoid the janky scrolling experience, the mouse event needs to happen elsewhere.

"What went wrong?" Microsoft explains. "Today, scrolling using the scrollbar takes place on the main thread. A long-running [JavaScript] can lead to mouse events getting queued up on the main thread event queue. This results in janky scrolling to the end user. For better user experience, scrolling using the scrollbar needs to move to the impl thread."

Microsoft's proposal would mean that when the main thread is busy, users can continue to scroll using scrollbars. 

A Chromium developer politely told Microsoft the issue that was reported is not a bug but a "feature request". Nonetheless, the Chromium developer agreed to pass it on for closer inspection.   

"Thanks for filing the issue," the Chromium developer replied. "As the issue seems to be a Feature request, hence marking it as Untriaged and requesting someone from respective team to look into it." 

Microsoft says it has already kicked off discussions with a Chromium developer about the feature.

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