Screens/ScreenTabs: do not pass event codes to isalpha()

[prownd]

The select-by-typing fallback in ScreensPanel_eventHandlerNormal(), ScreenTabsPanel_eventHandler() and ScreenNamesPanel_eventHandlerNormal() guards only the upper bound:

if (ch < 255 && isalpha(ch))

but the value reaching a panel event handler is not always a character. Panel.h defines EVENT_SET_SELECTED (-1), EVENT_PANEL_LOST_FOCUS (-2), EVENT_HEADER_CLICK(x) (-10000 + x) and EVENT_SCREEN_TAB_CLICK(x) (-20000 + x), and ScreenManager_run() dispatches all of them to the focused panel; ERR (-1) can arrive as well. Only some of those are matched by an explicit case, the rest fall through to the default branch.

C11 7.4p1 requires the argument of the <ctype.h> functions to be representable as an unsigned char or equal EOF, so this indexes glibc's ctype table far out of bounds. It is not merely theoretical - a standalone reproduction of the same expression:

ch=-2        table index -2        isalpha()=0
ch=-10000    table index -10000    isalpha()=0
ch=-19997    table index -19997    isalpha()=1

the last one making htop call Panel_selectByTyping() with -19997.

Use the same guard the other panels already use:

if (0 < ch && ch < 255 && isalpha((unsigned char)ch))

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 763e498 and 0003295.

📒 Files selected for processing (2)

• ScreenTabsPanel.c
• ScreensPanel.c

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📝 Walkthrough

Walkthrough

The tab and screen name handlers now call Panel_selectByTyping only for positive alphabetic characters below 255. Control and non-letter characters no longer trigger type-ahead selection. ScreensPanel_eventHandlerNormal now casts the character to unsigned char before calling isalpha.

Change	Result
Type-ahead guards	Accept alphabetic characters only
Character classification	Uses an unsigned char argument

Priority: ⬇️ Low

Change: Bug fix

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_{Letters pass through the panel gate
Control keys now wait
Type-ahead follows a clearer path
Unsigned characters avoid mishap
Three small checks keep input straight}

_{Comment @coderabbitai help to get the list of available commands.}

[BenBE]

LGTM.