[wasm] Drag gestures starting from above/with existing selection are not captured correctly

[Xiaoy312]

Current behavior

On WebAssembly, when there is selection underneath or around a control that typically uses drag gesture (eg: ScrollBar or Slider), the drag gesture will only work for the 1st frame/tick/delta, subsequent pointer capture events are lost, and there is an undesired browser dragging preview following the cursor:

Expected behavior

^ probably shouldn't happen? at least, there should be an easy way to disable this behavior, whether this would be disabled by default is subject of discussion

How to reproduce it (as minimally and precisely as possible)

<StackPanel Spacing="16">
    <TextBlock Text="selectable text: // select from here" IsTextSelectionEnabled="True" />

    <!-- some controls that typically use drag gestures -->
    <Slider />
    <ScrollBar Orientation="Horizontal" IndicatorMode="MouseIndicator" />

    <TextBlock Text="selectable text: // to there"  IsTextSelectionEnabled="True" />
</StackPanel>

• goto: https://playgroundcanary.z19.web.core.windows.net/#f68b7cf6
• select from the 1st textblock to the 2nd textblock
• try to drag with the slider's or the scrollbar's thumb
• notices the drag work very poorly

Workaround

from the javascript side, call e.preventDefault() in the pointerdown event handler of any impacted control

just run this command from the dev console of the browser:

document
    .querySelector('[xamltype$=ContentControl]')
    .querySelectorAll('[xamltype$=ScrollBar], [xamltype$=Slider]')
    .forEach(x => x.addEventListener('pointerdown', e => e.preventDefault()))

Works on UWP/WinUI

No

Environment

Uno.WinUI / Uno.WinUI.WebAssembly / Uno.WinUI.Skia

NuGet package version(s)

Uno.WinUI@5.5.37

Affected platforms

WebAssembly

IDE

No response

IDE version

No response

Relevant plugins

No response

Anything else we need to know?

No response

[dr1rrb]

Unable to repro neither on FF nor Edge on my side

FF : 132.0 (64 bits) Edge : Version 131.0.2903.51 (Version officielle) (64 bits)

[dr1rrb]

Humm, actually able to repro ... but on edge only!