fix(web): fix construction for suggestions emitted after left-deleting whitespace 🚂

[jahorton]

Also adds unit tests to cover the most substantially affected method. That method also had its parameter tweaked to better reflect the data it actually needs, which simplifies what must be provided for unit tests.

Build-bot: skip release:web release:android

User Testing

Use Keyman for Android for the following user tests.

TEST_ROBUSTNESS: Spend at least 5 minutes trying to "break" predictive text and/or cause it to work improperly while using sil_euro_latin and gff_amharic. Report back on any issues discovered.

TEST_REPRO_1: Attempt to reproduce the error-case reported in a user test on #16223:

1. Using the gff_amharic keyboard
2. Clear any pre-existing context
3. Apply the left-most suggestion; it should be ላይ
4. Type ውስጥል (Positionally, QWERTY xskl.)
5. Tap to apply the highlighted suggestion
6. Type q (result: ፅ)
7. Backspace over q
8. At this step, the typed text should look like ላይ ውስጥም
9. The left-most word suggestion should be the reversion appearing as "ውስጥል" + other suggestions
10. Do not apply the reversion, instead apply the second-left-most unhighlighted suggestion; it should be ፅኑ
11. ISSUE: At this step, the typed text should look like ላይ ውስጥምፅኑ (no space between ውስጥም & ፅኑ)
12. EXPECTED: Shouldn't the text result be ላይ ውስጥም ፅኑ (a space between ውስጥም & ፅኑ)?

Screen.Recording.2026-08-04.at.11.51.21.in.the.morning.mov

TEST_REPRO_2: Attempt to reproduce the error-case reported in a user test on #16223 (using gff_amharic):

1. Clear any pre-existing context (In the video, I did not)
2. Type ውስጥል (Positionally, QWERTY xskl.)
3. Tap the highlighted suggestion
4. Apply one of the default suggestion; it should be ላይ
5. At this step, the typed text should look like ውስጥም ላይ
6. Type f
7. Notice that the typed key on the keyboard is still highlighted/pressed down because of the different colors
8. Backspace until the cursor have reached the whitespace after the ውስጥም
9. ISSUE: No reversion.
10. Backspace once more for the cursor to land after the word ውስጥም
11. ISSUE: The suggestion word is ውስጥም instead of ውስጥል
12. EXPECTED: The reversion ውስጥል should display for this.

Screen.Recording.2026-08-04.at.11.51.57.in.the.morning.mov

[keymanapp-test-bot]

User Test Results

Test specification and instructions

• ✅ TEST_ROBUSTNESS (PASSED) (notes)
• ✅ TEST_REPRO_1 (PASSED) (notes)
• ✅ TEST_REPRO_2 (PASSED) (notes)

Test Artifacts

• Android
  • KeyboardHarness apk
  • KMSample1 apk
  • KMSample2 apk
• Web
  • KeymanWeb Test Home

[Meng-Heng]

Test Specs

1. Keyman for Android v19.0.259-alpha-test-16307
2. Android Studio: Emulator Piel 9a API 37

Test Results

• TEST_ROBUSTNESS (PASSED):

1. Clear any pre-existing context
2. Using the sil_euro_latin keyboard
3. Try producing errors by using backspace, tap, space to apply and delete the applied suggested words
4. Using the gff_amharic keyboard
5. Try doing the same as the step 3
6. VERIFIED: The predictive text does not crash or produce any noticeable errors.

• TEST_REPRO_1 (PASSED):

1. Using the gff_amharic keyboard
2. Clear any pre-existing context
3. Apply the left-most suggestion which isላይ
4. Type ውስጥል (Positionally, QWERTY xskl.)
5. Tap to apply the highlighted suggestion
6. Type q (result: ፅ)
7. Backspace over q
8. Text result: ላይ ውስጥም
9. VERIFIED: The reversion is "ውስጥል" + other suggestions
10. Do not apply the reversion, instead apply the second-left-most unhighlighted suggestion which is ላይ
11. VERIFIED: The text result is ላይ ውስጥም ላይ
12. Alternatively, applying the reversion does work as expected.

• TEST_REPRO_2 (PASSED):

1. Using the gff_amharic keyboard
2. Clear any pre-existing context
3. Type ውስጥል(Positionally, QWERTY xskl.)
4. Tap the highlighted suggestion
5. Apply one of the default suggestion which is ላይ
6. Text result is ውስጥም ላይ
7. Type f
8. Backspace until the cursor have reached the whitespace after the ውስጥም
9. VERIFIED: The appropriate reversion shows up as the word suggestion + other suggestions.
10. Backspace once more for the cursor to land after the word ውስጥም
11. VERIFIED: The suggestion word "ውስጥል" + ውስጥም

[ermshiperete]

I don't understand this comment. Does it explain the if (!penultimateTransform) ? In that case it would be better if it would come before the break; line. Or does it refer to the next if?