Javascript HTML选择块滚动条

我有一个选择块，看起来像这样

问题是它只有右滚动条，大多数选项太长，框的大小必须这么大。我想要一个向下的滚动条有什么办法吗？我更喜欢CSS或简单的HTML更改，不想让JS参与其中，但也可以打开所有选项，干杯

<select name="unselectedojects" size="10" id="selobjectlefttorightid" style="width:270px" multiple="multiple"> 
    <option value="selobject439">Alnwick English National Concession</option> 
    <option value="selobject418">Bedford English National Concession</option> 
    <option value="selobject652">Bedford Extra Local Concession (Disabled)</option> 
    <option value="selobject653">Bedford Extra Local Concession (Over 60)</option> 
    <option value="selobject423">Bedfordshire English National Concession</option> 
    <option value="selobject440">Berwick-upon-Tweed English National Concession</option> 
    <option value="selobject441">Blyth Valley English National Concession</option> 
    <option value="selobject512">Bournemouth & Poole English National Concession</option> 
    <option value="selobject452">Bracknell Forest English National Concession</option> 
    <option value="selobject453">Buckinghamshire County English National Concession</option> 
    <option value="selobject419">Cambridgeshire County English National Concession</option> 
    <option value="selobject233">Cheshire LM Multi Leg Journey</option> 
    <option value="selobject230">Cheshire LM Period Pass</option> 
    <option value="selobject224">Cheshire Multi-journey</option> 
    <option value="selobject223">Cheshire Period Pass</option> 
    <option value="selobject281">Cheshire Private Application</option> 
    <option value="selobject221">Cheshire STR Scheme</option> 
    <option value="selobject474">Christchurch English National Concession</option> 
    <option value="selobject475">Cornwall County English National Concession</option> 
    <option value="selobject443">Darlington English National Concession</option> 
    <option value="selobject429">Derbyshire County English National Concession</option> 
    <option value="selobject477">Devon County English National Concession</option> 
    <option value="selobject472">Diamond Travel Card English National Concession</option> 
    <option value="selobject620">Diamond Travel Card Extra Local Concession (Disabled)</option> 
    <option value="selobject621">Diamond Travel Card Extra Local Concession (Over 60)</option> 
    <option value="selobject478">Dorset County English National Concession</option> 
    <option value="selobject444">Durham County English National Concession</option> 
    <option value="selobject531">EC ISL Private Application</option> 
    <option value="selobject427">East (reserved1) English National Concession</option> 
    <option value="selobject428">East (reserved2) English National Concession</option> 
    <option value="selobject522">East (reserved2) English National Concession Local Enhancement</option> 
    <option value="selobject479">East Devon English National Concession</option> 
    <option value="selobject626">East Devon Extra Local Concession (Disabled)</option> 
    <option value="selobject437">East Midlands (reserved 1) English National Concession</option> 
    <option value="selobject523">East Midlands (reserved 1) English National Concession Local Enhancement</option> 
    <option value="selobject438">East Midlands (reserved 2) English National Concession</option> 
    <option value="selobject499">East Riding of Yorkshire English National Concession</option> 
    <option value="selobject507">Greater London (reserved1) English National Concession</option> 
    <option value="selobject529">Greater London (reserved1) English National Concession Local Enhancement</option> 
    <option value="selobject508">Greater London (reserved2) English National Concession</option> 
    <option value="selobject506">Greater London English National Concession</option> 
    <option value="selobject517">Greater Manchester English National Concession</option> 
    <option value="selobject456">Hampshire County English National Concession</option> 
    <option value="selobject645">Herefordshire  Extra Local Concession (Disabled)</option> 
    <option value="selobject646">Herefordshire  Extra Local Concession (Over 60)</option> 
    <option value="selobject487">Herefordshire English National Concession</option> 
    <option value="selobject421">Hertfordshire County English National Concession</option> 
    <option value="selobject457">Isle of Wight English National Concession</option> 
    <option value="selobject476">Isles of Scilly English National Concession</option> 
    <option value="selobject510">Kent County & Medway English National Concession</option> 
    <option value="selobject500">Kingston Upon Hull English National Concession</option> 
    <option value="selobject430">Leicestershire County English National Concession</option> 
    <option value="selobject431">Lincolnshire County English National Concession</option> 
    <option value="selobject422">Luton UA English National Concession</option> 
    <option value="selobject545">Merseytravel Disabled "Access all Areas" Concession SET</option> 
    <option value="selobject546">Merseytravel Elderly "Access all Areas" Concession SET</option> 
    <option value="selobject424">Norfolk County English National Concession</option> 
    <option value="selobject448">North East (reserved1) English National Concession</option> 
    <option value="selobject449">North East (reserved2) English National Concession</option> 
    <option value="selobject524">North East (reserved2) English National Concession Local Enhancement</option> 
    <option value="selobject501">North East Lincs English National Concession</option> 
    <option value="selobject643">North East Lincs Extra Local Concession (Disabled)</option> 
    <option value="selobject644">North East Lincs Extra Local Concession (Over 60)</option> 
    <option value="selobject502">North Lincolnshire English National Concession</option> 
    <option value="selobject636">North Lincolnshire Extra Local Concession (Over   60)</option> 
    <option value="selobject635">North Lincolnshire Extra Local Concession (Over 60)</option> 
    <option value="selobject488">North Warwickshire English National Concession</option> 
    <option value="selobject450">North West (reserved1) English National Concession</option> 
    <option value="selobject525">North West (reserved1) English National Concession Local Enhancement</option> 
    <option value="selobject451">North West (reserved2) English National Concession</option> 
    <option value="selobject503">North Yorkshire County English National Concession</option> 
    <option value="selobject622">North Yorkshire Extra Local Concession (Disabled)</option> 
    <option value="selobject623">North Yorkshire Extra Local Concession (Over 60)</option> 
    <option value="selobject432">Northamptonshire County English National Concession</option> 
    <option value="selobject637">Northamptonshire County Extra Local Concession (Disabled)</option> 
    <option value="selobject638">Northamptonshire County Extra Local Concession (Over 60)</option> 
    <option value="selobject433">Nottingham English National Concession</option> 
    <option value="selobject539">Nottinghamshire County English Concession (Disabled)</option> 
    <option value="selobject538">Nottinghamshire County English Concession (Over 60)</option> 
    <option value="selobject513">Nuneaton & Bedworth English National Concession</option> 
    <option value="selobject634">Orphan IPE for Transient Ticket Log</option> 
    <option value="selobject459">Oxford English National Concession</option> 
    <option value="selobject435">Peterborough UA English National Concession</option> 
    <option value="selobject2687">Scottish National Concession Blind plus Companion (Test)</option> 
    <option value="selobject2685">Scottish National Concession Disabled plus Companion (Test)</option> 
    <option value="selobject2693">Scottish National Concession Ferry Carnet Type 2 (Test)</option> 
    <option value="selobject2676">Sottish National Concession Disabled plus Companion</option> 
    <option value="selobject2692">Sottish National Concession Ferry Carnet</option> 
    <option value="selobject2674">Sottish National Concession Over 60</option> 
    <option value="selobject2677">Sottish National Concession Visually Impared</option> 
    <option value="selobject2678">Sottish National Concession Visually Impared plus Companion</option> 
    <option value="selobject470">South East (reserved1) English National Concession</option> 
    <option value="selobject526">South East (reserved1) English National Concession Local Enhancement</option> 
    <option value="selobject471">South East (reserved2) English National Concession</option> 
    <option value="selobject462">South Oxfordshire English National Concession</option> 
    <option value="selobject649">South Oxfordshire Extra Local Concession (Disabled)</option> 
    <option value="selobject485">South West (reserved1) English National Concession</option> 
    <option value="selobject527">South West (reserved1) English National Concession Local Enhancement</option> 
    <option value="selobject486">South West (reserved2) English National Concession</option> 
    <option value="selobject465">Vale Of White Horse English National Concession</option> 
    <option value="selobject447">Wansbeck English National Concession</option> 
    <option value="selobject494">Warwick English National Concession</option> 
    <option value="selobject466">West Berkshire English National Concession</option> 
    <option value="selobject497">West Midlands (reserved1) English National Concession</option> 
    <option value="selobject528">West Midlands (reserved1) English National Concession Local Enhancement</option> 
    <option value="selobject498">West Midlands (reserved2) English National Concession</option> 
    <option value="selobject495">West Midlands English National Concession</option> 
    <option value="selobject467">West Oxfordshire English National Concession</option> 
    <option value="selobject505">West Yorkshire English National Concession</option> 
    <option value="selobject654">West Yorkshire Extra Local Concession (Disabled)</option> 
    <option value="selobject484">Wiltshire County English National Concession</option> 
    <option value="selobject657">Wiltshire County Extra Local Concession (Disabled)</option> 
</select> 

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请看：嗨，问题901089中的解决方案不起作用，我的问题与此不同one@ManuelvanRijn我讨厌人们在没有真正阅读帖子的情况下留下评论