Structured Enter and Backspace need editor owned keydown paths

What happened

The first Android split/join proof row exposed a nasty failure shape:

• collapse selection inside a structured bold text run
• press Enter twice
• press Backspace twice
• expect the original paragraph text to come back

Instead, both Android Appium and desktop Chromium collapsed to:

• One before two middle

That proved the row was not only an Android transport problem.

Root cause

Current slate-v2 was barely owning structural keyboard editing there.

Plain text insertion had good browser/input coverage, but plain unmodified:

• Enter
• Backspace
• Delete

were still falling through to browser DOM churn instead of the editor’s own structural operations.

For this row, browser DOM split/join behavior drifted far enough that commit from DOM lost the suffix.

What fixed it

Route the unmodified structural keys through editor operations in Editable:

• Enter -> Editor.insertBreak(editor)
• Backspace -> Editor.deleteFragment(...) when expanded, otherwise Editor.deleteBackward(editor)
• Delete -> Editor.deleteFragment(...) when expanded, otherwise Editor.deleteForward(editor)

Keep modifier variants out of scope for this narrow fix.

That was enough to turn the same semantic row green on:

• desktop Chromium
• Android Appium Chrome emulator

Why this matters

This is exactly the kind of gap that looks like “Android weirdness” until you force the same sequence through desktop and see the same break.

The useful move is:

• classify the row as a product-bug candidate
• fix the shared editor-owned path
• only then go back and judge the device-specific result

Reusable rule

If a browser/input row depends on structural editing, do not trust browser DOM default behavior to preserve the model.

Own the structural key path in the editor first, then measure platform quirks on top of that.