Screen reader - block count incorrect after inserting a block

[microbit-robert]

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Description

After inserting a statement block, the screen reader output for the total number of blocks in the stack is incorrect by +1 (i.e., it says 3 of 4 when there are only 3 statement blocks). This is observable when re-selecting the newly inserted block . The new block can be inserted anywhere in the stack. Note that the block position and count of the surrounding statement blocks is correct.

The incorrect values are fixed / updated when a different block change is made elsewhere.

We thought this might be fixed when #9295 was addressed, but it appears to be a separate issue.

Reproduction steps

1. Use https://google.github.io/blockly-keyboard-experimentation/screenreader/
2. With a screen reader switched on, insert a block into one of the existing c-shaped blocks
3. Select the newly inserted block and observe that the total block count is incorrect by +1

Stack trace

Screenshots

Screen.Recording.2025-09-24.at.11.04.09.mov

The inserted draw heart block is announced as "2 of 3".

Browsers

No response

[BenHenning]

We can probably consider this resolved with #9395 since removing tree information will presumably make this redundant (though we could have an underlying counting issue we may run into with the other block labeling work).