fix(whiteboard): defer invalidate to vsync to avoid flicker

[hdream0322]

Purpose / Description

fix card flikering issue while using whiteboard (legacy study screen)

Fixes

• Fixes Card flickers when drawing on whiteboard #19364
  (same issue occurs from recent version to 2.21, so I think it's webview issue)

Approach

Force the Whiteboard view to a software layer so its invalidate calls don't disrupt the Webview's GPU.
Webview still on hardware.

How Has This Been Tested?

Tested on Samsung Galaxy TabS9+, Android 13 (SDK 33), WebView 148.0.7778.120.

Steps to reproduce:

1. Use the legacy review screen
2. "Disable card hardware render" off
3. Open a Basic-template card and enable the Whiteboard.
4. Draw short strokes on the card.

Before: card content flashes white during short strokes.
After: no flicker. (Long strokes, single dots, eraser....)

Screen_Recording_20260514_144014_AnkiDroid.mp4

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[welcome]

First PR! 🚀 We sincerely appreciate that you have taken the time to propose a change to AnkiDroid! Please have patience with us as we are all volunteers - we will get to this as soon as possible.

[hdream0322]

sorry, my mistake

[david-allison]

TL;DR: Let's use git bisectfirst

Thanks for the PR!

This implementation causes a performance regression. Given we know this was introduced between 2.23 and 2.24, we should understand the bug before fixing it, so we can fix it in an effective manner

Given you can reproduce this on your phone, and I can't, please perform git bisect to find the commit which introduced this issue. From there, we can put in a targeted fix.

If you need anything, please let me know

• See: #19364 (comment)

[hdream0322]

Dug into this some more. The invalidate() calls in handleDrawEvent have been there since 2021, so it's not really a regression — and on my phone every older release flickers too, so I couldn't bisect locally.

But @ZornHadNoChoice confirmed 2.23.3 OK / 2.24.0 NG on Xiaomi, so there's clearly some environment trigger I just can't repro here.

Ran a bunch of layer-type combos on upstream/main to pin down the mechanism:

• WebView with SW layer → no flicker
• Whiteboard with SW layer (the old fix) → also no flicker
• Whiteboard with HW layer set explicitly → flicker, AND in that case the color-picker / split-screen workarounds stop helping too

That last one pretty much confirmed it for me — the trigger is the synchronous invalidate() on the touch path racing with WebView surface composition.

So instead of going back to the SW-layer workaround I switched the stroke hot path to postInvalidateOnAnimation() — same effect, vsync-coalesced, no layer type changes, no perf hit.

Would really appreciate it if someone on the affected side (Xiaomi folks especially) could confirm this fixes it for them too.

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Since this is my first PR, there may be some rough edges or things I've missed. I'd really appreciate it if you could point them out — any feedback would be very helpful. Thank you so much for taking the time to review.