diacritic: Critical bugfixes

commit c72ffa7a9ae5b5da7533a84d45fd91ee22ddc72f
Author: Solomon Peachy <[email protected]>
Date:   Mon Mar 9 21:40:40 2026 -0400

    diacritic:  Critical bugfixes
    
     * When the codepoint > 0xffff, don't overflow past the end
       of our diacritic table (Fixed by William Wilgus)
     * Truncation of the 'info' field causes the RTL flag to be dropped
       (RTL flag is b15 but we truncate it into an 8-bit variable)
    
    Both bugs introduced in a2c10f6189 (September 2025)
    
    Change-Id: Id5425606f2cf91d3b3a81f4b67a97d546de81e41

diff --git a/firmware/common/diacritic.c b/firmware/common/diacritic.c
index 68c8dcd4c9..878eba1064 100644
--- a/firmware/common/diacritic.c
+++ b/firmware/common/diacritic.c
@@ -203,9 +203,15 @@ bool is_diacritic(const ucschar_t char_code, bool *is_rtl)
     static uint8_t diacritic_mru[MRU_MAX_LEN];
 
     uint8_t i, itmp;
-    uint8_t info, mru;
+    uint8_t mru;
+    uint16_t info;
 
-    const struct diac_range *diac;
+    const struct diac_range *diac = &diac_ranges[DIAC_NUM_RANGES - 1];
+
+    /* If the codepoint exceeds the terminating entry in our table
+       then treat it as non-diacritic. */
+    if (char_code >= diac->base)
+        goto bypass;
 
     /* Search in MRU */
     for (mru = 0, i = 0; mru < mru_len; mru++)
@@ -244,6 +250,8 @@ Found:
     diacritic_mru[0] = i;
 
     diac = &diac_ranges[i];
+
+bypass:
     info = diac->info;
 
     /* Update RTL */
-- 
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