User Agent: Mozilla/5.0 (Windows NT 6.1; rv:76.0) Gecko/20100101 Firefox/76.0

Steps to reproduce:

1. Run the attachment.
2. Click on the button (id="button_container").
3. Change button_container's size to various sizes.
   examples:
   style="width:100px;height:100px;position:absolute"
style="width:20000px;height:50000px;position:absolute"

Actual results:

When clicked on button_container, there's almost no lag when button_container's size is small, and huge lag when it is large, and the color hangs on orange for a while.

Expected results:

No lag. Not even when there's fast consecutive clicking of button_container. Chrome doesn't have this issue, even with fast consecutive clicking.

Removing filter:url(#filter950) in button_foreground's style makes the lag disappear.

Bugbug thinks this bug should belong to this component, but please revert this change in case of error.

If you turn off webrender does this still happen?

@Robert Longson, yes it still happens. It was turned off by default. WebRender doesn't seem supported:

Beslissingenlogboek DIRECT2D
> +------------------------------------------------------------------------+
> | Op blokkeerlijst; foutcode BLOCKLIST_FEATURE_FAILURE_DL_BLACKLIST_g984 |
> +------------------------------------------------------------------------+

D3D11_HW_ANGLE
> +----------------------------------------------------------------------+
> | Geblokkeerd vanwege bekende problemen[bug                            |
> | 1153381](https:\/\/bugzilla.mozilla.org\/show_bug.cgi?id=1153381)    |
> +----------------------------------------------------------------------+

WEBRENDER
> +----------------------------------------------------+
> | opt-in by default: WebRender is an opt-in feature  |
> +----------------------------------------------------+
> | available by user: Force enabled by pref           |
> +----------------------------------------------------+
> | unavailable-no-angle by runtime: ANGLE is disabled |
> +----------------------------------------------------+

WEBRENDER_QUALIFIED
> +------------------------------------------------------+
> | blocked by env: Battery Intel requires os compositor |
> +------------------------------------------------------+

WEBRENDER_COMPOSITOR
> +------------------------------------------------+
> | unavailable by env: No DirectComposition usage |
> +------------------------------------------------+

OMTP
> +---------------------------------------------------------------------+
> | broken by runtime: OMTP is not supported on 32-bit with \<= 2 cores |
> +---------------------------------------------------------------------+

ADVANCED_LAYERS
> +--------------------------------------------------------------+
> | available by user: Enabled for Windows 7 via user-preference |
> +--------------------------------------------------------------+

WEBGPU
> +-----------------------------------------------------------+
> | disabled by default: Disabled by default                  |
> +-----------------------------------------------------------+
> | blocked by runtime: WebGPU can only be enabled in nightly |
> +-----------------------------------------------------------+

I can also reproduce on MacOS 10.14 with and without WebRender enabled.

mstange: Maybe you have an idea what's going on?

The lag comes from the computation of the blur in software.
When WebRender is enabled, we have a fast path for SVG filters that are only a blur, but only if the x and y blur radii are the same. The filter in this testcase has stdDeviation="3.01 3.51587". Changing that to stdDeviation="3.51587" makes the testcase much faster for me when WebRender is enabled.

Seems OK now.